However, the greatest growth since his first appearance in Australian colours in 2010, at 18, where he stood on the blocks a shade over six feet tall and weighing 88kg, was between his ears and deep beneath the muscle and sinew which, at Magnussen’s best, propels him up and down a pool at lightning speed.

Now, 22, he’s been hardened by realising in the wake of the London Olympics that swimming was more about the mettle within than the public’s demand for gold medals. He says the scars from a Games, where he left Australia hailed a hero but returned seemingly cloaked in a nation’s disappointment after finishing with ‘‘only’’ a silver medal in the 100m freestyle event, have healed. However,  recalling how he felt when his effort; attitude; desire was being questioned by people who didn’t know him appeared to sting.

‘‘The race itself, I can’t really remember at all,’’ he said. ‘‘The aftermath was stressful and it took a lot of getting over. Looking back on it now I’m pretty proud of how I handled that week and the person it’s made me since. I think more than anything I see that [time] as a growing experience, a learning experience.

‘‘Yeah, I probably went pretty close [to snapping]. At the time it felt like a roller coaster out of control. From that relay [where the Aussies were favourites to win but finished fourth] everything went crazy. All of a sudden the media was hounding me for negative reasons a couple of days before the biggest race of my life.

It’s a whirlwind in my head, I can barely piece together the days. The people around me did the best they could to shelter me from all that but it spiralled out of control and it was hard to perform, but I’m proud of how I performed under the duress; I could get a silver medal. I think it was a testament to [coach] Brant Best and my support team’’

Magnussen overcame the fallout to successfully defend his world 100m championship crown last year. He grew three centimetres when he had six weeks respite from the chlorine and discipline after his Olympic campaign – ‘‘it was as if my body said thanks for the rest’’ – and his recent gym work has resulted in him looking like one of the Canterbury Bulldogs he’ll cheer on during the NRL season.

‘‘My body is changing and adapting as an athlete as I get older,’’ he said. ‘‘I’ve dropped two seconds over my 50m freestyle, and that’s a result of the power and the explosiveness I’ve developed. In turn, that really helps my 100m because I can get out faster in that first 50 with natural speed.

‘‘It also gives you a confidence getting on the blocks and knowing you can match it physically with the best in the world. When I competed at the Pan Pacifics in 2010, I was 88kg and I stood next to [Americans] Jason Lezak who was 34 and 100kg and Nathan Adrian – the best part of 100kg – and I felt small. Men’s 100m freestyle, it’s a macho event. You need to be confident in your skill, your ability and body.’’

Born in the NSW north coast township of Port Macquarie, Magnussen said he was, by nature, laid back but he detests the notion he and his teammates were pampered.

He recalled training day-in, day-out regardless of the weather in Port Macquarie’s unheated outdoor pool by muttering ‘‘character building’’. He’s a perfectionist and training can be pure torture if he thinks there is a slight imperfection in a stroke. When he moved to Sydney, at 18, with a mate from Queensland, they set up digs in a 180-year-old house in Lidcombe  where they stretched their meagre incomes by living on two-minute noodles and other rations which were as nutritionally poor as they were cheap.

‘‘Our first electricity bill,’’ he said. ‘‘We didn’t know you had to change the name the old electricity account was in. We thought we had a three-day-long blackout before we realised we hadn’t paid the bill that was in someone else’s name. We seemed to learn everything like that by making mistakes. That year I went to India for the Commonwealth Games I was probably the only guy on the team who didn’t get sick and I attribute my iron stomach to our cooking.’’

It was from such an environment Magnussen was thrust into Olympic favouritism – and the spotlight – after he won the 100m sprint at the 2011 World Championships. He went from being the kid who didn’t know the machinations of paying the household bills to being the face of Australia’s 2012 Olympic campaign and, just like the electricity bill saga, he learned on the run.

‘‘I’m still working to adapt to it but I’ve almost got the hang of it now,’’ he said of recognition.

‘‘It is a massive shock to the system to go from anonymity to people stopping you in the shops and it can be unsettling to know people have all these expectations for you. That was one of the toughest things I needed to deal with going into London.

‘‘I’m a laid-back person and I like to go to training, swim and leave that at the pool. I don’t spend my day talking about swimming, my performances or anything like that because it’s not who I am and that public recognition, the exposure, it constantly reminded me about swimming. If I was down the street, people would say ‘good luck’ and while the intention was good, it sticks in your head – Olympics. In the 12-month build-up you start to think about it day in, day out ...  and that was really hard because I was so used to leaving the swimming side of things at the pool but there I was, carrying it around 24-7 and it really weighed down on me.’’

His manager Mark Jones, of The Sports Group, said while Magnussen had a suite of sponsors as a result of  being the back-to-back world 100m champion, he’d pulled back on his outside commitments to allow him to have a ‘‘balance’’.

‘‘Since London we’ve reduced the commitments to allow James a little bit more to make sure he has that right balance in life and so he can apply himself to swimming to achieve what he wants to achieve,’’ Jones said.

The Olympic campaign proved even tougher than the build-up. A bonding session by the men’s 4x100m freestyle relay squad which involved the use of the banned sleeping tablet Stilnox was a scandal and the performances of the male swimmers was scrutinised. Magnussen was a focus of both stories.

Two-years on, his philosophical approach about the silver medal his family has under lock and key – because it symbolises to them his sacrifice, dedication and achievement – proved his transformation from boy to man.

‘‘It’s set me up for longevity in my career,’’ he said. ‘‘Had I won that Olympics, or broken a world record in the race, I would’ve ticked every box in swimming in the first three years of my career. It’s given me something to chase, it’s given me a hunger and determination.

‘‘I have well and truly moved on.’’