This is less likely with a naturally small mandible as opposed to one that has been surgically reduced.
Agreed. A reduction like the one above would result in sagging, especially the chin. For whatever reason, the tissues of that region just don't snap back after reducing. Intervention is necessary from what I have read and was told.
On the topic of natural jaw size and aging, I tend think larger jaws
can be aging, or, rather, they often don't age as well. There are exceptions, of course.