I remember reading about this. You basically have a set amount of fat cells. Your body fills these things like potato bags. When required to use, it empties some of the bags.
Also, your skin has a "memory" system. So when it damages, it knows how to rebuild it. When you lose a lot, the skin doesn't remember how to fix it properly. Of course you can reprogram it to fit, but losing an excess of 100s of pounds would require surgery. I think age has something to do with it too. If you're younger, you'll be able to fix it. But if you're older, it's going to be hard to fix it.