If you give the browser a form, it's going to want a callback(JS); to make sure everything went through. When you give it the callback, it'll probably ask you for a image; pretty UIs are important. When you give a browser an image, it'll need to be styled(CSS). . . and so begins the theory of "If you give a browser a cookie".

Granted 2MB is pretty big, but the only people I see complaining are techies; valid complaints but do not effect Twitter's bottom line.