GSP is a very intellegent person/fighter who I respect. But GSP/AS size difference advantage/disadvantage is simply just a bunch nonsence. Kenny Florian, a natural 145'er, competed at Welterweight as well as MW and has fared just fine. Jon Fitch, someone who GSP has been successful against, went 10-2 at Light Heavyweight before dropping down to 170. Vitor Belfort, a natural 185'er, was Heavyweight tournament winner, AND Light heavyweight champion before competing at middleweight. When all of those factors come into play, there is absolutely no excuse for why GSP, who is a proven natural welterweight, can't fight Anderson Silva at 185, or even a 180-178 catch weight. The size difference isn't too great, and would most likely offer a minuscule advantage that wouldn't have any effect on the outcome of the fight being if they were the same size.
Kenny Florian was hardly a "natural 145'er" he was a huge Featherweight.