Ive noticed a few things from females....
1. If they see you with a fat chick, they may think that you arent all about looks and that makes you look like a better person on the inside..Eventually girls choose that type of person over a meat head that just looks out for thin bimbos...
2. Girls are very competative, sometimes more so than guys. If they see you with a fat chick, they'll want a piece of that. Many times they sit there and wonder what he sees in her. Its enough to atleast get the thin girls attention. Girls are perceptive, a hot thin girl who takes care of herself all too much will take notice when a guy chooses to talk to an overweight girl instead of Ms. Vain....