Most fat/bbw sites don't even use counters so it's hard to tell.
1. Chubbyland
2. Plumprumps
after that it's hard to tell.
http://www.pornresource.com/tgps/bbw/
This listing is based on overture searches for the site name. It's not exactly accurate though as I don't think they lump in all the various search phrases that could be used.