Of course size plays a role, but I'm not going to just attribute everything Canelo did in that fight to a 10lb weight advantage, it's simply an ignorant way to assess a boxing match.
When I saw Canelo do this:
and this:
I didn't think 'oh well that was expected, Canelo is bigger' - that would be beyond ignorant. What you see in the above GIF's and most exchanges in the fight is superior boxing skill, and there's no dressing it up. better punch selection, accuracy, speed and timing. Those uppercuts are much more to do with Canelo having the best uppercut in the sport, than him having a size advantage.
I mentioned to Slim and Nac about the left hook. Canelo's size didn't help him defend the left hook, he defended it by holding his right glove close to his chin. Here' an example of what I was referencing, a blocked power shot from Cotto and a very accurate left hook response from Canelo:
In the instances where Canelo backed Cotto up and put some damage on him then yes, it is more than fair to argue that Canelo's size helped him out there. However, the majority of the fight was contested in the middle of the ring and Canelo was happy to box with Cotto and counter him hard, if anything that is a strategy that suits the smaller, lighter fighter who has the better jab.
There were so many exchanges in this fight that showcased Canelo's superior boxing skill, it really does baffle me that someone can review the fight and conclude that Canelo was simply 'bigger and fresher', and not mention the fact that Canelo was the vastly superior boxer in 8/10 exchanges. Those extra pounds don't make Canelo punch more accurately than Cotto, they don't help the timing of his defense and they don't make him a better counter puncher. If Canelo had Cotto pinned down for the entirety of the fight I could see the argument, but it was boxer-puncher style fight in the middle of the ring and outside of the jab, Canelo was sharper absolutely everywhere.
If you are talking about this fight and not mentioning the stuff I covered above and simply focusing on size then you are being disingenuous to what actually happened. You can mention how size played a role, but there's a big AND that needs to follow.