I tend to believe it is the main reason. Power changes everything. Nole cannot dominate the rallies or change the direction of the ball as he usually can against a good Stan. Take away Stan's power, what is he left with really? He does not have an extreme spin or anything that would bother Nole. He hits a very heavy ball and it takes Nole out of his comfort zone, in my opinion.
With
@atttomole's original post in mind as well:
I think is a mix of power, ability/confidence to change the ball direction, and a level of unpredictability. Actually I wanted to post a thread about the differences between Nadal and Djokovic regarding defense. Obviously Djokovic is quick, but not absurdly quick, or not 15 majors quick. What he is much above the rest (IMO), is a combination of court positioning and ability to read the opponent's shot. Of course his reflexes are also almost super human as well, but it is funny that he does not hit that many running forehands (way less than Nadal, for example). Why? Because he found time enough to position himself better and hit the shot with some balance. So he is used to have that fraction of extra time to set up his shots. Wawrinka's combination of power and unpredictability take time away from him.