Because if people were using that kind of huge avatars, loading a page with 20 posts would mean downloading 2 Megabytes in avatars alone. It would cause the forum to be very slow for users who doesn't have very high bandwidth, and increase the load on the forum servers considerably.
I don't know why *any* picture that size *ever* would need to be over 25K. But perhaps that's because I started making web sites in the era of 14.4 K modems, and 28.8K was considered very fast when it arrived... At that time you tried to keep pictures below 10K, and not more than two pictures on a page.