you guys are all wrong. it has nothing to do with them being fat. there are various features that are associated with fatness. there are various features that are are associated with sexual openness. some of these overlap, like heavy-lidded eyes, full lips, bulbous septums. note also that two of those three are more common with alcoholics. people are attracted to each other based on their features but they also become signifiers. remember the "low income white girl eyes" tiktok thing.
this is why a white guy can usually tell with considerably greater than random accuracy whether a white girl has been with a black guy, same for a black guy guessing about a black girl with a white guy. but its much more random for men guessing about women of other races. and i imagine the same pattern would hold for women.
so if you think all fat girls like black guys you probably just are more often attracted to and paying attention to fat girls with heavy-lidded eyes, full lips, etc. aka what youre really noticing is that people who look more sexually open also look more sexually attractive because youre been reinforced into being attracted to those signals