Well, I am not a physiologist, but from what I see big bulky muscles do not help with velocity. If you look at most big league pitchers, they do not have really buff arms. I think that muscle density makes it harder to completely relax the arm and get a "whip-like" action toward the plate. An old pitching instructor used the analogy of getting hit with a big piece of plywood or a whip. Obviously the whip is moving much faster.
I would check out Ross Enamait's site:
He uses a lot of bodyweight and other excercises to maximize strength and overall fitness.