With anorexia, when the person can actively be seen showing ED body checking and and doing other anorexic 'showy' behaviors such as dressing in such a way to show off or flaunt as much of their skeketal frame as possible, then it might be considered 'promoting' anorexia. It's not just a 'skinny girl,' it's an emaciated woman dressing in such a way to proudly show off her 'handiwork.'
Fat people get demonitized when they are shown doing muckbangs and pigging out, essentially binge eating on camera. Binge eating is a known ED, so it gets flagged.