I dunno... I'm not a big guy myself, but I see plenty of bigger guys at the gym. He's not really all that large, I think it's something that's pretty achievable by non-steroid methods. Consider that he probably didn't work out at all in his younger days - that would make the difference more noticeable, since most people who start serious lifting from being non-athletes are going to have an immediate/more obvious change as opposed to those with prior athletic background.
Besides, we already know Petrucci has a hell of a work ethic.