It isn't that he isn't big enough. That's not the reason.
The reason is the way he plays. He plays every run like he is running through a brick wall. It's one thing to do that 15 times a game. The body takes a beating, but not too much of one.
But if you start doing that 30 times a game, his body won't hold up to that abuse. There is a reason that runners who play like that don't have the longest career.
Look at Earl Campbell. That's the way he ran, and he had a dramatically shorter career than other great runners.