Aayush dislikes this behavior, but I think it's great. The varying sizes added a layer of visual complexity that just didn't need to be there.
That said, one thing I would really like to see in Expose is the ability to assign a unique "look" to a particular window so that the window can be found without much thought. For example, I usually have a "main" browser window with a billion and one tabs, and then various other "ancilliary" browser windows. It would be nice if I could give the main window a bright yellow color (or whatever) when in Expose so that I could jump right to it.