The maximum size is the same for every member now, and I really doubt Premium avatars will get a size increase as Jordan has stated there haven't been plans to do it.
Also, when uploading jpeg images as avatars, the file size drops considerably. As does the image quality. Before the update they would retain their size and almost png-level quality. So long as it was under the 25KB limit. I'm guessing that's because they aren't converted to gifs when uploaded here anymore?
EDIT: You can't even upload an avatar via online URL anymore? Huh.
And since the size limit is now 200x200 because profile pics and avatars are now tied in with each other, it would help greatly it the file size limit went up in conjunction with it. 5-10KB more would be perfect.