I would be reluctant to lend support to any increase in the size limit. Make the limit bigger, and the avatars will get bigger, you can be quite certain of that. Several times I've hit the file size limit when making an avatar and had to re-examine my file format (PNGs can be terrible sometimes) to make the avatar fit. If I hadn't hit the limit, I would not have bothered. Multiply me by a few hundred members, multiply each member by a few hundred posts and you have a serious bandwidth waste.
It's not just modem users either: I'm on cable but I still pay per MB. In any case, animation (while cute) can also be an eyesore. Think of the 6k limit as a challenge.
In short, while there is a case for bigger file sizes, I think the case for no change is a good deal stronger.