FYI, this is not something I did, and I'm not aware of which admin or moderator did do it, but I have some guesses as to what happened.
Recently, there have been a lot of complaints (I receive and read all the "bad post" reports, but in almost all cases let the other moderators deal with them) over "pornographic" imagery in user avatars, and there have been a small number of repeat offenders.
I wouldn't surprise me that as a result of this, the avatar file size limit has been reduced in an attempt to limit the porno avatars, nevermind that this solution doesn't really address the problem.
But I can state what has happened at a technical level. The file size limit has been reduced, but that limit is only checked when a new avatar is uploaded. If you have a larger avatar you'll (probably) be able to keep it as long as you want, but if you want to change it, you'll be stuck using the smaller size.
Unfortunately, this seems to be a case where a few bad apples have spoiled things for everyone else.
It is ridiculous to choose a solution which punishes everyone for the discretions of a few and does not even remotely solve the problem. So - now if someone was so inclined they have to post smaller porno?