Yeah, Pac didn't KO JMM. That's how good (or lucky) JMM was but Pac's succeeding fights proved he can KO even bigger boxers than him!
Floyd's brittle hands will not allow him to score a KO at WW. Floyd knows it better than anybody else, he's not punching hard enough to gain the respect of the big guys at WW that's why he has been avoiding them. Pac is smaller than Floyd but the Cotto fight showed that he could take a WW punch and win a WW fight by KO/TKO.
Its more about Floyd not being able to stop Marquez, not even being close to that ... you know.... a big guy is not being able to KO the tiny one, so he can safely repeat the same with the other tiny guy who seems to have much better chin and can punch harder.