Now that Pacquiao is weighs so much in these fighters, he is virtually the same weight as Mayweather when he fights, but Pacquaio is weighing that much after only 24 hours of hydration. Floyd is around that weight as well even though he fights at 147 he could step on the scale fightnight and virtually weigh the same as the night before. I think the size differential is not really there, and Pacquiao probably hits harder than Mayweather as a "130" pounder than Mayweather does at 147. IMO this would be a fair fight in terms of size.

Height: 1 1/2 inches for MAyweather
weight: The same considering Pacquaio wouldn't have to cut
arm lenght: 1 inch for Mayweather

I really don't see this being that unfair of a fight if you are talking about size. Oscar De La Hoya is way bigger than he was at 130, but Mayweather isn't really a true 147 pounder, or 140 pounder, perhaps his size still seems natural for somebody at 135, and thats where Pacquaio should be fighting at the very least.