GSP's skillset would definitely enable him to compete with bigger opponent's, even control the fight like he does with other WW's, but whether he could BEAT them in an MMA match is a different story which is indeciferable until it happens.
Could he control a bigger man until the final bell, without taking heavy damage? Could he actually finish a bigger man? The latter is much harder to imagine, given his recent lack of finishing ability at WW.
Remember that we all train with people of all different shapes and sizes, and I'm 100% sure GSP is comfortable with a larger opponent. But ultimately, when it all comes together on fight night, you don't want to be fighting a bigger, stronger, heavier, yet faster man who is just as athletic as you and is more accustomed to that weight than you are. This is the situation GSP would be faced with against Silva and I think he'd get handled.