yes, that could simply be it. I only mention it as I think the added muscle and size was part of making Loki seem more powerful than he was in Thor 1, and so more acceptable that he'd be a serious challenge for all those heroes in Avengers, but if size = power and he's since lost that muscle mass, then it would seem he's less powerful then. No?
FYI, Tom Hiddleston appearance Dec. 15 to open the Dragon Christmas Charity sale. http://www.dragonsale.org/