It really is just the final 1-2%. What builds the muscle is adequate nutrition and training. The pros know how to train for growth stimulus and they can rest better. Also, their genetics are superior and they have the dedication most trainers lack.
The drugs are a tiny little element - in fact, there is a growing belief that pros are better off saving the money from buying expensive crap like HGH, Slin, AAS and spending it on stuff that actually makes a difference: protein isolate and creatine.