She actually talked about it at one point: when she was much younger she was pretty overweight, and while she was able to work it all of naturally (with enough time and support) she was left with the excess skin. That is also why she's got the stretch-marks she does: she had gained a lot of that weight fairly suddenly as a kid (can't remember the specifics, I think it was a change in medication for something), so even when she lost the weight it wasn't like everything magically shrunk back down (movies are full of lies).