Is there a way to stop my avatar being forced into a 125x125 square, even if it's smaller than that?
Since my 80x80 avatar looks just fine - are you linking to your avatar via the URL field in the settings?
If so, the forum has no way of telling just how big your avatar is, and will just add a width and height of 125 to the image tag for it. If you upload it to the forum, on the other hand, it can examine the image and use the correct size.
np: Cotti - The Beach (Soul Jazz Records Singles 2008-2009 (Disc 1))