For those who haven't watched early MMA, miss the classic technique over strength lesson in fighting. An undersized Royce Gracie proved this point by winning multiple UFC tournaments against much heavier competition. There have been many classic David versus Goliath match-ups in mma where the smaller yet more skilled fighter dominated. Now looking back into the past, those match-ups really don't seem all that miss-matched because it is now a well known fact: if you are a novice white belt going against a high level Jiu-Jitsu black belt you don't stand a chance.
Floyd isn't that much heavier than Ronda. And for those saying Floyd's power and strength will be too much for her, look back to Jason Miller versus Stefan Gamblin. Miller was giving up over 190 pounds yet finished the bout in less than 30 seconds. Why wasn't power a factor then? For those saying Floyd's boxing talents alone will be enough, rewatch Randy Couture versus James Toney.
The truth is in the past.
And this goes beyond fighting, in any sport, just because you are a man, it doesn't give you the ability to understand a sport better than someone who has studied and practiced it for her entire life. With that said, Floyd Mayweather doesn't stand a chance against Serena Williams in a tennis match. He'd get beat by Danica Patrick in a car race. And he'd get arm-barred in an MMA match against Ronda Rousey. This matter isn't simply a man against a woman, it's an amateur against a professional.