The main problem is actually the textures, which don't compress very well in PNG. My original idea was to give the minimap a "hand-drawn" look, but Jorb insisted on this "illuminated manuscript" look. The current minimaps look very nice, I'll agree, but "hand-drawn" ones would probably compress far better.
It would help using JPEG, of course, but, well... blech.
Anyway, the server has a much larger hard drive now, so it shouldn't be a problem for a long time. I have also written a program to GC old, unused minimap images; I just couldn't use it previously since the filesystem turned out to be using non-indexed directories.