Wing Chun is a very fast martial art, and he practiced it a lot, and i don't believe he was on steroids, look at his body, very low body fat, but a very good muscle to weight ratio. I'm 14 in like two days, and am muscly and well cut for my age, but he weighed slightly less than me. I'm also about the same height as he was - Around 5"7. And he was much stronger than i am now, and his muscles are bigger than mine, and i'm not fat, this just shows that he really did get his body fat right down. If you see him in Enter the Dragon in the first fight scene, then he looks very skinny (and muscly). I believe that he wasn't very well at this time though, and wasn't eating like he should have.
Be like Water