It use to be smaller...450x110 I think (boy was that hard to work with). After much out cry it was upped to 500x115. The point was not to clutter threads up with large images but also allow GFX people more room to work with. Hince 1 image and the new size.
While I know of sites that allow for bigger images as well as multiple sig images I can't say that I like the idea. As it normally ends up with over half the post being taken up by the sig area which is not good for reading through posts.
Your suggestion however, isn't too bad as long as the new size stays less then 150 pixels IMO, more then that and well...clutter heaven. Just my 2 cents