Death Note yaoi: Why is mello always an uke compaired to Matt?

Hm .... good question. Even though he may be clified as a uke compared with Matt, doesn't mean he's way submissive. He's probably just secretly uke.(; Haa, a very controlling uke ... And I think it's because he's more feminine in body structure. I also think he's shorter than Matt ... ukes are always shorter.^^