Someone already said it, and it's culture. Our culture idolizes the traditional Stalone type character, the typical steroid enhanced MANLY man. I think we do this because we have always held the idea that strength comes from physical perfection. Eastern cultures often rely more heavily on the whole inner strength thing, mostly because of their religious beliefs I would assume. So when you look at it, their heroes don't have to be that statuesque muscle bound idiot that Western heroes almost always have to be. Of course there are always exceptions, like Gatts or Riki-Oh for example (great, ridiculously powerful Japanese heroes) and...well come to think of it I can't think of any non-muscle bound bad-asses in western culture, except for like spider-man or batman, and even they are always portrayed as ridiculously well built, just not as broad shouldered or bulky.
