I read an article once that said that losing muscle helps to cause the loose skin and lots of exercise, drinking Water, and TIME will help to bring the skin back. It said the skin needed time to recover just like any other organ. It may mean that some people's skin just takes more time to heal than others. It also mentioned that there was still fat deposits left inside the skin and that's why more exercise is needed.
I don't remember where I read it but I remembered that part of it. I think the link was once posted here on the forum.