Genetics are pretty much everything in almost every sport... I knew a guy like that, he was about 6'6, 250lbs or so freshman year in high school! He ate nothing but crap, everyone bitched that he never did the workouts right in practice, that he was lazy, but yet he was lean enough to step on stage...
This always talked bad about steroids and as did the other athletes... I don't really follow sports except for boxing... But he went on to play college ball and in the pros... His name is Simeon Rice... I also went to school with Donovan McNabb, he was the quarter back, but only after my class left, he graduated a year after our class; but of my friends was actually a way better Quarter back than Donovan, so he always started before and Donovan didn't get a chance to play until our class left... Ironically Mike the quarterback at our school didn't want to be a pro football player and focused on premed I think he went to Northwestern... But everyone was offering him football scholarships (His team won state all four years he was there, he was in sports illustrated and they also beat a few powerhouse teams from different states) That's pretty admirable, I don't know a lot of people who would've given up a chance like that to stick to the books...
I know McNabb got pretty famous; because I used to see him on commercials when back when I watched TV... It was a very small school, they put an addition on it about 18 years ago and it started going to shit...
I think we had a class of less than 200 senior year and it produced a lot of athletes, two national wrestling champs, one wrestled in the Olympics, Antoine walker the basketball player and the football players I had mentioned...
The point is, that all of these kids looked the same in high school as they did a decade later playing professional sports... The wrestler Joe Williams, didn't change a hair on his head, same height, weight, was always ripped since freshman year in high school... They had the look of thoroughbreds since age 15 or so...
I don't know for a fact; but I'd be willing to bet that all of them stayed natural... So it's believable that someone could have that much muscle naturally, as some have other talents naturally; it's just rare...