I don't think the body size matters.

I think it's down to other elements here.

I feel Pac is a bit more well rounded fighter then Cotto.
While Cotto does have a better jab, Pacs got speed and vice versa they both have things they are better then the other.

In the end I think Cottos too patient and is too relaxed someone like Pac who punches in bunches and is busy throughout the fight can overwhelm him and pin him on the ropes.

I also believe Pac has the power to hurt, drop and possibly TKO, Cotto.