Something to keep in mind is that most people don’t wait for their weight to stabilize before posting photos like that. Weight loss can be hard on the body, especially when it’s relatively fast, but that doesn’t mean that it can’t be recovered from while maintaining the new weight. Often within even a month or two, the remaining body fat will begin to redistribute in a more typical pattern (I feel like one can see this particularly in many people’s faces, which can look a bit gaunt during active loss) and the skin will start to adjust to its diminished size. This is of course a different thing than people who genuinely end up underweight in a medical sense, but there too our perception of what constitutes normal/healthy can be badly skewed by the environments we’re in, and it’s possible that the person deliberately set the photo up to look as “thin” as possible and it’s not entirely representative of how they’d appear in person.