That's my point. Let me tell you guys how this would go... because it's gone this way EVERY time a seasoned boxer has fought a veteran MMA professional....
Mayweather would come to the middle. Rousey would go low and shoot the leg out from under him. With no experience on the ground, Mayweather would have no idea what to do, and down he goes.
Rousey goes for the mounted position. Mayweather tried to throw a punch from his back, something he has never learned to do (and it is different). Rousey grabs the arm, and it's arm bar time.
Mayweather taps.
Floyd wouldn't be the first boxer to think he can handle himself in an MMA bout. The 2nd UFC featured a former champion wearing one boxing glove... no idea why. It always ends the same way.
Down they go. They try to punch. Joint lock of some form or another... and tap.
Mayweather's only hope would be to spend a year learning jiu jitsu from a Gracie, 12 hours a day every day, before the fight to get comfortable in the guard, the mount, etc...
Put them in an octagon today, right now, and he's tapping. Put them in a ring, and she's knocked the fuck out. But on her terms, there is no way he wins. Zero. His chances are nil.
If he was 220, sure, because now she grabs that leg and maybe he doesn't go down. Maybe he stands up long enough to throw a knockout shot. But at his size, leverage wins, and he goes down, and then she owns him.