"She never puts on any weight, you'll notice that's often true of selfish women." -- Joan Didion, Play it as it Lays
It's part of the job of novelists or their characters occasionally to throw out such over-generalizations. Ideally they ring somewhat true, or at least tell us something about the narrator.
From David Foster Wallace's story "Westard the Course of the Empire Takes its Way" --
"D.L. was severely thin, thin in a way that suggested not delicacy but a kind of stinginess about how much of her she'd extend to the space around her. Thin the way mean nuns are thin."
Maybe Wallace encountered thin mean nuns at some point in his life, but I suspect he drew this image from the well of his television-watching experience.
Claims about thin women tend to strike me as ideologically suspect.
From Betsy Lerner's Food and Loathing -- "A high school friend's father once said that losing weight was merely a matter of vanity. Thin people, he said, were more vain and cared more about their looks. The comment has stayed with me. I've searched my soul and I promise you: I'm vain. I'm Carly Simon vain. If it were just about vanity, I'd be Kate Moss."