Not sure if he is on something but don't rule out him being on absolutely nothing.
Some folks just have great genes (such as myself).
He actually isn't that big. Look more into body changes like (david boston ohio state vs nfl david boston) or Jamal Lewis at Tenneesee running style was more of a TB and then in the NFL he blows up and turns into a powerback damn near breaking the nfl rushing record for a year .....and now all of a sudden the bastard can barely maintain 70 - 80 yrds a game (HMMMM
?? that's a huge drop off for a young back that only 3 years ago cracked over 2 grand on the ground and was damn near unstoppable)