If you think Mayweather weighs below 150lb during a fight then you must be naive. When he fought Marquez, everyone knows he weighed over 150 lbs. I am guessing 155 but he didn't even want people to know how much he weighed.
Mayweather weighed 150lbs the night he fought De La Hoya. If you think he's ever come close to weighing 160lbs for a fight like the thread starter, there is no helping you understand the facts.
Mayweather chooses not to reveal his weight because of the big discrepency so there is no way to tell. Mayweather looks way bigger than Pac when he fought Marquez and Mosley.