It's lipedema, which is a disorder where fat seems to be very preferentially stored in the hip, butt and thigh regions and it's very hard to lose weight from them, even if the patient loses weight through diet/exercise to the point the rest of them is underweight. This one is quite lucky in that the distribution isn't as squicky looking as some others which can look very puffy and complicated by lymphedema(excessive fluid accumulation). Glitterandlazers has the same condition except that it's a lot larger than this dumpy chick pictured here.