It's about 95% genetics. If your dog is destined to be ripped and muscular, he will be. If not he's not pre-disposed to it and you try to make him look that way anyway, you'll just end up hurting him.
I like the analogy that an APBT should have the build of a lightweight or welter-weight boxer: compact, lean, toned and wirey. Game-bred (aka fighting) APBTs are endurance athletes more than anything and they should look the part.
These things you see looking like Arnold Swarzenegger are mutts, plain and simple.