That much better or that much bigger?

[CFH]

I think it was a bit of both.
Mayweather would have beaten Marquez at any weight in my opinion, and pretty easily too. Sure, the weight didn't help Juan, but the fight was not a physical one. Mayweather was pot-shooting all night and not imposing any physical advantages he had. Marquez looked to struggle with the extra weight a bit, but again, the fight was not high tempo and Marquez has never been known for explosive power and speed.
I think stylistically, Marquez is made for Mayweather. Two great counterpunchers, but one is in a league of his own. Mayweather's reflexes were simply too quick for Juan and his counters too accurate. It would have been the same no matter what weight they fought at.

Marquez is a gutsy dude though, he never loses heart. He impressed me even though it was pretty much a shut out.
Mayweather is something special though, like him or not, he is that good.

This.

I would say both.

Mayweather was simply too big and too good for Marquez.

And this.

It was both.

If they were level weights Marquez wouldn't have looked so outclassed but would have still been outboxed. Simple as.

And also this.