It's always amusing how laypeople make medical diagnoses based on a single photograph; I know plenty of girls and women who are naturally skinny but eat healthy diets and exercise.
But back on topic. Joe's measurements get to the bottom of it. Fashion shoots and especially ads have very little to do with real-world measurements or average proportions. There's
this recent more blatant example, then there's
this clip showing how an exceptionally pretty lady is taken to whole another level in post.
There was an online article a couple of months back where one post-processor for the biggest high-end clients talked about his trade, which was very insightful. This gentleman had very little interest in retaining any semblance of realism in his subjects. Unfortunately I wasn't able to locate the article with quick googling.
I wouldn't be surprised if the shot is a composite since it seems to be a fad after Annie Leibovitz's Lavazza ads. But most of the otherworldly nature of the shot comes from turning her body into one with Martian proportions.