If your that good it won't matter. Gsp is one of the bigger WW's. Did it matter when Cain beat a 40 pound heavier Brock? Or what about Edgar fighting guys with 20-30 pounds on him? If he's that's good he would beat silva.
I'm sorry, if you think size doesn't matter, there isn't much to say. It doesn't matter if a black belt is fighting a white belt, but it sure as hell matters if a black belt is fighting another black belt. The first line of your post is basically proving my point. Size doesn't matter when there is a huge skill discrepancy, but when there is no skill discrepancy, or a small one, it is a huge
Or do you think Brock Lesnar still beats Randy Couture if they are the same size?
And the Velasquez fight proves my point to a tee. That was the fight where a lot of people really started to realize how much Lesnar relied on his size and how profoundly awful his striking was. Every fight Lesnar won, he won solely on size, the only reason Valasquez could
beat him, was because he was a WAY better fighter than Lesnar. If Lesnar was anywhere near Cains skill level, he wins every time. And when Lesnar fought a guy close to his size with a decent skill set, that was Overeem, and Lesnar got obliterated.
People have mentioned the BJ-GSP fights, and those are another good example. I happen to think GSP has a better skill set anyway, but it didn't matter if there is a little difference one way or the other, it would only matter if there was a lot of difference. GSP is too big for BJ. BJ never wins that fight, and size is a huge part of that.
Weight classes matter, and matter a lot, that's why all combat sports have them.