I agree with Coach on this one. My son is one who throws hard for his size. He is 5'9", 155 lbs and this summer got to where he cruised at 84-85 and was hitting 86. He said he felt like he wasn't throwing that hard. I have seen guys much bigger than him not throw that hard. We have had a number of people tell us that he doesn't even look like he is trying.
In one of his last pitching lessons at the end of January, he had a guy who played AAA stand in while he threw to see his stuff. He tried to jack a couple of pitches in harder than usual and dropped 3-4 mph and was not as accurate when he did it. He learned a big lesson that day.
Anyway, I think using your whole body including core and legs in unison is what helps get the velo up there.