My guess is they say they like fat chicks since that's all that's they can get.  Bullshit yourself long enough, you start to believe it.  They say they only bang fat chicks because that's what they want to do.  Rather than accepting they're settling.    Kinda like all those fat chicks that say they love being fat and love their body, but then throw shade at any skinny and attractive women they see.  Lie to themselves long enough, they actually start to believe they prefer to be this way, rather than being one of those "skinny bitches." They're a Real Womyn™ instead.  Denial is a hell of a thing.