there is a SKILL difference.
go watch old GSP interviews. it is clear he once wanted to challenge champions of higher weight divisions. now, it is clear he isn't interested in any of that. why? because as good as he is, he knows he is not the p4p #1. and NO, the problem is not size. and it is not striking.
GSP has very good striking.
GSP has excellent bjj.
GSP has absolutely amazing wrestling. he WOULD take down silva with ease. rashad evans claims gsp takes HIM down with ease ffs.
however...
GSP lacks head movement. for all his jabfest acting as active striking defense, for all his moving in and out to dodge strikes, GSP still lacks specificaly head movement. its not that he has a bad chin-his chin is good. but whenever he gets hit, he gets hit clean .
GSP, for some reason, cant target the chin. he targets the eyes, or the face, or the nose, whatever. but you never see him going for the chin! he obviously aint that accurate. he just "aims high".(imo; i could be wrong, but thats my opinion). this explains why he doesnt have enough KOs, even pre-serra, despite his obviously enormous power that breaks bones with jabs(koscheck) or almost lifts middleweights in the air(see jab to shields).
and that my friends, both the capacity to target the chin to go for the kill, AND (most importantly) head movement to avoid getting hit clean, is skill disparity. and the true reason why silva is the p4p GOAT and not gsp. not generic striking(gsp is good. silva is amazing, but the difference aint huge due to takedown threat), not bjj(gsp is better imo), not takedowns(size?what size? you seen how athletic gsp is and how lightning-fast and technical his double-leg is?)
sad part is... i kinda suspect both these weaknesses have to do with st pierre's striking style(kyokushin karate), which isnt very good for head movement/punches to the chin, due to punches in the face being disallawed overall. its not realy st pierre's fault. just bad habits.
