I think ignorant webmasters lump them all into that category, and i'm sure the ugly ones love to put themselves in that category, but that doesn't make it so. If you check out the websites that are actually doing well in the BBW niche, they are actually pretty girls. You do get people that want the really horrid looking fat women too, but most people don't seem to know the difference and that's where the problem is.
Take Mandy Blake. GFY's very own BBW. I'm not into large women, so she doesn't do it for me.. but just looking at her, you can see in her face that she is pretty.
http://www.mandyblake.com/bbw-mandy-blake-welcome.htm
Whereas the girls on say chubby.com... ehh... just fat.