The thing about health and skinny/fat: any extreme position is wrong. Some people's bodies just "want" to be a shape that's not viewed as ideal by society at the moment. People's "set points" for fat level vary widely, and you can eat in a healthy way and be at your set point and not risk unhealthful outcomes and still be too fat or too skinny by society's aesthetic standard.

For most people, the set point is in a familiarish place. It's almost certainly a standard distribution type of situation, where the set-points for roughly two thirds of people are in that zone that agrees with health-insurance company charts about BMI and whatnot. And some people for whatever reason of genetic heritage or personal uniqueness are farther out in one of the tails, rounder or skinnier that the two thirds.

Society's aesthetic standards vary with time, but the conditions for human health don't. The desired shape of women varies even faster than desired BMI (crinolines, bustles, the long-neck period, etc.)