As far as for me, Auto Bar can jump up to 36 megs. After a garbage collection it drops back down to 3-4 megs. Sit around idle, go afk, check it again, 24 megs. Garbage collection, back to 3 megs.
I too only have a few custom buttons, but eve so, it shouldn't be using this much memory. I'm posting on the Auto Bar comments too, but thought I'd post here as well.
Screenshot of 36 meg usage:
http://images.wowstead.com/10855/192.jpg