From reading stuff on here and watching videos people post here from TV shows about fat people, I'm going to guess that there's a bunch of fluid accumulating around that area. So gravity pulls it down and stretches the skin, giving a place for more fat to go, and more fluid builds up in it, etc. After a certain point I'm sure the fat is just looking wherever it can to go, since the body is just so packed with it. It really is mindblowing though that its leg somehow managed to develop its own fupa.