Do guys like thin girls?
Most men like a healthy looking women with good skin tone and a body that looks fit. The biology around bearing healthy children comes jnro play here. The package that shows health can exhibit itself in all kinds of shapes and sizes. But in general a slim woman is going to be healthier than a woman that is too thin or overweight.
Heart disease is often a silent killer, especially in women. It sneaks up on us as the symptoms are more subtle than those in men. There are also a lot of other diseases that are correlated to being at an unhealthy weight thin or heavy. Even out of child bearing years, most partners, men and woman, don't want to have to become caregivers to those who have physical issues because they aren't taking care of themselves.