You have me thinking in another direction on what causes muscle mass, and I'd appreciate some input!
Marathoners have small legs. Sprinters have huge thighs. A lot of powerlifters are not big guys...but they can outlift many very large bodybuilders.
I'm thinking that muscle size is correlated to time under tension. If the time is too long (like a marathoner) the muscle doesn't grow. Too short, like the guy doing 1 rep of a squat, and there is some growth, but nothing like the 6 plus rep guy.
Or could it simply be a training balance between fast and slow twitch muscle fibers?