At 1.80 m (5 ft 11 in) with shoulder length hair and a permanent smile, the Montana native appears decidedly smaller, and perhaps more easy going than most of his giant, crew cut counterparts.
But Miller also differs from his peers in that he swims with a noticeably reduced lung capacity - the result of a condition known as pectus excavatum.
The American's "sunken chest" means he is swimming with 10-20% reduction of his Vo2max, in a sport where breathing efficiency is everything.
“When you look at me from the side, it looks like I have a big hole in my chest, because I do.
"I think around age 10 is when it kind of started to develop,” the 28-year-old told Olympic Channel in an exclusive interview.
For an active kid participating in a sport where most of the body is on display, having a reduced lung capacity wasn’t even the worst aspect.
The physical appearance of his chest caused Miller social anxiety.
“The hardest part wasn't so much knowing that I don't breathe as well as some people, but was just just being really self-conscious about the way that I look,” he continued.
“I talk to kids all the time when I do clinics, about being comfortable in your own skin, especially with young girls. I know what it's like to walk into a room and not feel comfortable and not love the way that you look in the mirror."
“That was a real learning process for me, figuring out how to get over that." - Cody Miller on body confidence issues
“I'm not Nathan Adrian, I'm not Caleb Dressel, I don't have the chiseled six pack abs. I'll never be that guy. Once I accepted that I look kind of funny and that it was no problem, it stopped bothering me so much.”
There is a light-hearted joke in some sporting circles that swimmers can’t run, and don’t have the coordination to play ball sports.
Typically, this swimming stereotype couldn’t be further from the truth for Miller, who played several sports growing up including ice hockey, tee-ball, and skateboarding.
“I loved skateboarding. I have three longboards in my garage right now that I am not allowed to ride in case I get injured!" the Indiana University grad revealed.
“I wrestled a little bit my freshman year of high school and I was pretty good at it, not because I was really strong or anything, but because I had such great endurance from swimming. I could just outlast them.
“I don't like the stigma that all swimmers are really uncoordinated and not athletic because I don't think that that's true. If you take the US national team for example, over 50% of the guys on that list are very coordinated and very athletic. If we put a basketball in our hands, it would be a good game.”
The future Olympian’s sporting prowess didn’t finish there.
Miller also took two years of classical dance class at high school, to surely prove that swimmers not having coordination was a myth.
“I was not a good dancer!” he said with a smile. “But I did take two years of lessons because I liked a girl. I was the only guy in the class.
“I enjoyed ballet and I did like a lot of modern contemporary dance. I don't think they get enough credit as athletes. Dancers are so strong and committed and it’s incredibly hard."
To say that Miller is a huge movie and comic buff would be serious understatement.
Alongside his wife, the pair sometimes dress up as their favourite characters for a movie premiere at the cinema, and he carries an Iron Man figurine to his competitions as a good luck charm.
And just like the Marvel superhero, who also suffers from a chest condition, Miller’s weakness became his motivation to succeed.
He was also told that swimming had the potential to help.
“The doctors said that swimming was going to help broaden my shoulders.
“So if I never swam and my shoulders and chest didn't develop the way that they did, I might have actually needed a surgery.”
The affable American became a star in the water, and eventually had his Hollywood moment at the Rio 2016 Olympics.
He shocked the field to win bronze in the 100m breaststroke with a new national record, before teaming up with Michael Phelps, Ryan Murphy, and Nathan Adrian to take home gold in the 4x100m medley relay.
“People always ask me, ‘If you didn't have that [sunken chest], would you be faster? And I'm like, ‘I don't know, because I feel like part of it made me."
“I've always had this chip on my shoulder, like I feel like I need to outwork my competition and I need to outwork people because I'm operating at a disadvantage. And that's just always kind of been a part of me.” - Cody Miller to Olympic Channel.
In truth, there is more background to Miller’s performance in Brazil that makes his achievements there even more impressive.
His father, who was once on course to play professional ice hockey in the NHL until his career was ended by a serious knee injury, suffered from alcoholism and drug addiction most of Miller’s life.
Just six months before the USA Olympic Trials, the swimmer - who was now living in Las Vegas with his mother - received a phone call from a detective in California to say that his father had been found dead.
“We found out he had died on Christmas morning,” Miller recalls.
“I always describe it as like the scene of a war movie when the grenade goes off and everyone's shell shocked, it's ringing, and time kind of stands still for a second. That's how I felt. It was very bizarre."
But Miller harnessed the power of his mind to focus on swimming, and to not let the tragic news derail his Olympic journey.
He vowed to process his father’s passing after the Games, and this mental fortitude became a hallmark of his character.
“It was horrible. It was tough. I went through it. But on the other hand, this may sound odd but it made me really resilient.
“My ability to compartmentalise as an athlete and block things out stems all the way back from when I was 13 trying to avoid things that were happening with my dad, like seeing him go through withdrawals, having seizures and stuff like that." - Cody Miller to Olympic Channel.
“I think having that ability to shut everything else out is critical to high level performance at any sport. I was able to separate myself from reality or being able to be in the moment when I needed to."
At the US Trials he managed to secure his place on the plane to Rio 2016 with a second-place finish in the 100m breaststroke behind Kevin Cordes.
Both men went on to make the Olympic final in Rio. But while the cameras focused on Britain’s Adam Peaty after the race, who broke the world record to win gold, Miller could be seen in the background celebrating in utter jubilation.
The American had delivered the performance of a lifetime to win the bronze medal with a new national record.
“I had pre-race nerves and jitters and all that. But I really at the heart of it was much more at peace and calm at that meet than maybe ever before, because I was just very accepting of whatever was going to happen. I don't know, but that's how I felt... I was kind of Zen about it. I felt at peace.”
Despite his achievement Miller had to refocus, and quickly.
His next task in Rio was to swim the breaststroke leg for Team USA in the 4x100m individual medley final, where he would be locking horns with Peaty once more.
This in itself was a daunting enough task, but Miller had the additional pressure of not letting teammate Michael Phelps down in his final race before retirement.
“It was very surreal for me because I grew up idolising the dude,” Miller said of the 23-time Olympic gold medallist.
“I had posters of him on my walls as a kid. I basically worshiped the ground he walked on. I remember watching him on TV, and then fast forward 10 years later and I’m on his career-ending relay!
“Obviously I was swimming next to Adam Peaty again, and I knew how fast he was going to go. I was like, ‘just don't blow this.’ I just needed a solid split. That feeling of getting hunted down, which I knew was gonna happen, really sucks."
“It was a ridiculous amount of pressure, but at the same time we were going with four medallists, which no other team had.”
Recently crowned backstroke Olympic champion Ryan Murphy put USA in the lead, and despite another unbelievable swim from Peaty, Miller produced an excellent breaststroke leg to keep Team USA in the race.
This allowed Phelps to storm ahead in the butterfly, before former freestyle Olympic champ Adrian finished things off on the anchor leg, to seal the gold medal with a new Olympic record.
But the nature of the occasion, coupled with an under-powered meal before the race, left Miller totally exhausted afterwards to the point where he passed out.
“Most people don't know this, but I had a really small dinner that night as I was so nervous, and we were racing really late. It was literally the last race of the Games.
“I swam the race and like 10 minutes later, after we'd done a couple of media interviews, we're backstage getting ready to go on the podium, and I basically passed out because my blood sugar was so low. I was literally laying on the ground and I couldn't see anything. I was freaking out. That had never happened to me in my entire life!
“I didn't eat enough, but also the nerves and the adrenaline and everything just went through me, and we didn’t have a cool down recovery swim.
“They delayed the medal ceremony because of me because I couldn't stand.”
Unsurprisingly, Phelps’s brilliance in the water made a huge impact on the young prodigy.
Miller nostalgically tells a story about when he first met his swimming idol in the terminal of Washington D.C airport, aged 15, on the way home from a national junior meet.
The breaststroker learnt a lesson from Phelps that day that shaped the rest of his career.
“Freaking Michael Phelps walks up to me just in the airport and starts talking,” Miller recalled.
“They upgraded his ticket, and he made sure mine was upgraded too, so I got to sit up front and talk with him in first class for a few hours. This was months after he’d just won eight gold medals at the Beijing Olympics, and here he is just being nice because I'm clearly a little fanboy!
“I remember one of my dumb questions was, ‘How did you do it? How did you win all those medals? And he looked at me and said, ‘I recognise that I'm talented. I recognise that I have natural gifts, but above all, that I will outwork everyone if I'm determined enough’.
“That really, really stuck with me over the years. You have to have that mindset of just always wanting to do more, push harder, just kind of the no limits mentality.”
On top of the dedication to his craft, Miller is perhaps best-known for his positivity.
After the Rio Olympics he set up a YouTube channel, where he shared swimming drills, nutrition tips, and offered a fascinating insight into the life of an elite athlete.
The channel has been a booming success, and he enjoys almost 150k subscribers today.
His ability to turn any difficult task or practise into something fun is the resounding theme of the channel.
“I want people to feel that sense of joy and that sense of happiness and passion when it comes to the pool, because that's how I feel most of the time. It brings me back to when I was a kid.
“I've always been an optimist. I've always been positive, but I haven't always been overwhelmingly expressive about it. That changed after I achieved my lifelong dream at the Rio Olympics.
“Some days I'll wake up, I'll feel like crap, and like I don't really want to swim, and I don't really want to make a video. But on those days if you force yourself to do something and you put on a smile, it makes things easier. Then you see progress.
“A sense of positivity goes a long way.”
An example of Miller putting his positivity into practise in a tough situation occurred during the lockdown.
With the Tokyo 2020 Olympics postponed, and no nearby pool open to train in, he started practicing in a pond with teammates.
“We were just kind of doing it because there really was nothing else to do during quarantine. So it kind of refreshed a lot of our senses.
“We were doing it just solely because we wanted to do it. And that was really kind of nice and kind of fun.” - Cody Miller on quarantine training.
Coronavirus isn’t the only thing changing Miller’s preparations for the Olympics.
His wife is due to give birth to their son in November this year, which was supposed to happen after the Games.
But after the year-long postponement of the Olympics, Miller’s focus will likely be slightly less swimming orientated going into 2021, and he has just two goals.
“One make the team, and two, go best times. That's it. Keep it simple.
“Ideally I’d want to break 2:08 minutes [for the 200m breaststroke]. But let’s keep it simple. Make the team and just enjoy doing it too.”
It doesn't look like that ear-to-ear smile will be going anywhere, anytime soon.