I imagine that if her pants were properly fitted, that they might reveal her feminine body-fat distribution and skeletal structure, aka womanly hips. I'm pretty sure that's part of why she starves herself as well. The more body fat you have, the less sexually ambiguous your frame is going to be. Cross-sex hormones only do so much, especially if you aren't doing any other kind of body conditioning or modification. It's easier to look at least androgynous if you're a skeletor. It's the same reason why MTF Dylan Mulvaney starves himself. Though, in both cases, the ho-ho-Holocaust look also ends up having the opposite effect in certain ways. Being frail emphasizes Ellen's female smallness, and being frail emphasizes Dylan's male shoulder width and hand size. Basically, you can't win in the game of trying to pass as the opposite sex. The best you can hope for is losing less horrifically, and that usually requires really intensive and really expensive shit.