It's not his size. It's his training regimen; resistance and gymnastics which give him that incredible explosiveness. No wonder wrestlers can't stop his takedowns because he jumps into them using that forward momentum.
Here's the thing. Anderson Silva holds his own against LHWs Stephan Bonnar, Chael Sonnen, and Okami who's a very strong middle weight. He also trains with Lyoto, Big Nog, and JDS so Anderson is very accustomed to higher weight classes.
GSP's advantage is clearly his strength, agility, speed, endurance, reach (jab), and explosiveness combined together. Fighting Anderson his strength, endurance, reach will be negated thus his only advantage would be agility, speed, and explosiveness in the ground game. You can also say that Anderson is very quick which he is, more with his head movement and striking. GSP glides around the Octagon like a flyweight and covers a lot of distance. I think I remember him taking down someone from the middle of the Octagon all the way to the fence. GSP will have to expend far more energy keeping Anderson down.
As much as I want to watch this I think it's a really bad proposition for em. As I said in previous posts, I'm not sure he'll take it...even if he does...he won't seem pumped. Which is too bad cuz it's a very intriguing match up. If GSP had KO power or an iron chin (Hendo style) it would be that much more interesting.
When you compare all current and past champions they are all mortal. However one seems to be a cut up the rest.
Marcus Aurelius: Tell me again, Maximus, why are we here?
Maximus: For the glory of the Empire, sire.
Baked, not fried... the healthy choice.