That's not even true, though. It depends on a whole bunch of things like weight distribution patterns, body shape and genetics. For example, it's largely been demonstrated that a "thick" chick with a curvy body is likely to age better than a skinny chick with a "straight up and down" body. Because as the thick chick ages and gains weight, it is often deposited more in the hips, ass, thighs and breasts. So while she might end up "fat," she still has a curvy shape, for the most part.
When the girl is completely skinny with no hips to begin with, as she ages and gains weight, she just ends up getting... wider. (And it's often worse if she's skinny with huge tits) I think if you scroll through this post you'll see a lot of chicks who were skinny and ended up very fat.
In Skin Diamond's case, she was NEVER "skinny." She was slim-thick. She always had subtle curves. And she was also into health and fitness. So it's no surprise that she would retain her form more.
