But why even bring the fact of her fatness into the equation? Isn’t it irrelevant?
It doesn't seem so to me. She's gross and ugly, and a specific kind of crass. There are people who are, no doubt unlike any of your relatives, unattractive in the way they carry extra weight. In addition, all body types bring their special temptations. Among people who are fat, I've seen many of both sexes who aggressively crash their way through crowds. Here I see the latest in a long series of fat women who, perhaps in revenge on girls who more easily attract male attention, make a lot of obscene noise about sex. They attract attention to themselves and make everyone else, no doubt especially women, feel intimidated and self-conscious.
A big reason they often get away with it is that people are reluctant to call them on their malicious rudeness: Overweight carries a certain victim status among many bureaucracies. This is a corrupt kind of reluctance. Obscene is obscene, and the public square needs to be defended from it.