Apparently, you can be too thin in the fashion world, but it isn't a matter of health. It has more to do with the aesthetics of showing lots of skin with a tad too much rib.
"[The magazine] Numéro, it seems, liked that [top model Karlie] Kloss was so thin but hated that her thinness made her bones more apparent. So in a have-your-cake kind of gesture, it made the bones disappear with Photoshop. ... We've become conditioned to expect perfected images of skinny, apparently boneless, smooth little girls in our magazines. In a certain way, we've come to rely on Photoshop to insulate us from the sharp reality of what maintaining an industry-approved fighting weight can do to a human body."