The other thing that you have to consider is that in the end he wasn't really that big. He went from a totally emaciated state to having a decent physique. That emaciated state was not natural for him. So, his body was able to pack on muscle incredibly fast when given the proper food, training, and drugs. Not to mention that he's preparing for a role that's paying out the ass. He could devote all his time to training, recovery, and nutrition.