I fucking hate sizeism, and I hate when thinness is equated with attractiveness.
When unattractive people go on and on about how attractive they think they are, I think it's kind of funny. When attractive people do, I think it's shallow.
I'm happy when I can muster up legitimate vanity (because I used to hate how I looked), but I don't think it usually tends to go beyond "Holy shit, these pants *do* look good" (obviously in situations where applicable) or saying "I know" in reply to compliments (which frankly I do to be irritating as much as anything else).