I think the biggest thing is, Broner knows he has a good chin. I think Floyd must've learned early that he might not of had a great chin or his father beat it into his head that he'd better rely on having great defense.
I've always said that if Mayweather would've stayed at 130-140 for most of his career, he would've likely been in the top 20 fighters of all time in a lot of people's opinions. People would've griped that he was too big for those divisions at some point, and he probably wouldn't be a household name, but as far as legacy I think it would've helped. I think he could've easily dominated guys like Morales, Barrera, Casamayor, Freitas, Juarez, Rios, Hatton, Marquez, Juan Diaz and this current crop of 140lb fighters etc... Not to mention, the Pacquiao fight would've been more likely to happen I think because Mayweather wouldn't hold this superstars mentality that came with all that money and drawing power. He would've been worth less in terms of money, but worth more as far as legacy I would guess.