Freddie Roach knows boxing, and he knows Anderson Silva and Georges St. Pierre. Roach has spent time with both men, trying to aid in refining their boxing technique. The other thing that Freddie knows is superfights that don't come together, being one of the central players in the ongoing Manny Pacquiao vs. Floyd Mayweather, Jr. drama.
Those are just a few reasons to listen to Roach's reasoning for why he feels Georges St. Pierre vs. Anderson Silva is never going to happen:
Quote (transcribed by MMA Mania):
Anderson Silva hits harder (than Georges St. Pierre) because he's a lot bigger. But Georges is also a good puncher also. But the thing is, size wise, I think Anderson's a little more fluent at this point. He's a lot bigger than Georges, of course. That's why I don't think that fight, they've been talking about that fight happening, I don't think it will happen. I've talked to Georges about it and (Silva is) just way too big.