Few key differences.
In boxing weight classes are ALOT closer in weight.
If i remember correctly the weight difference was that of 6 pounds?? Pac was like 144 and mar was 150?? Iv seen people in MMA give out 30-40 pounds.
In boxing physical strength does not play nearly as big of a difference as it does in MMA. (Many fighters have said this, on top of my head i can think of Musasi) In MMA body to body contact is an extremely important aspect. In boxing it doesnt play nearly as big of a part for the sheer reason that the main aspect of boxing is punching and moving. So 15 pounds in boxing wont make nearly the difference then 15 pounds would if you have that person laying on top of you.
If GSP took the fight as quick as everyone wants him to then there is a good chance that Silva would be 20-25pounds heavier then GSP come fight time. If GSP did things the right way and take a year to bulk up. Anderson Silva would still have a healthy 5-10 pounds on him and lets not ignore the Height and reach advantage. GSP would have to swing UPWARD while Silva would be swinging in a perfect straight should length line. GSP's face could not be a better jab target.
ALL OF THIS means that GSP going up to MW is silly and pointless. I want to see GSP challenge himself by keeping the belt for another 5-10 years which could mean another 15-30 fights. Then retire on top.
THAT WOULD BE A LEGENDARY MMA FIGHTER. GO DOWN WITH THE GREATS LIKE BRUCE LEE!!