fat girls approaching you = you look too much like a nice guy, reek of "niceguy" etc etc.
You have to reek of "niceguy" in order for a fat girl to "woman up" and talk to you. This is because they have less fear of being put down. You think they are going to be approaching the jerk that will tell them to f___ off? they only approach guys they either see as feminine/weak/nice that they think are not as high up from their level.
For the sake of your reputation never be seen talking to a fattie...unless she is hanging around with 2 of her hot friends then I wouldn't mind saying hi to her in order to get an introduction to her pals