Well, the droopy appearance of folds of skin is most probably caused by crash diets that sacrifice large amounts of supporting muscle, or lean body mass. Even less severe diets and weight loss from bariatric surgery can result in a substantial amount of muscle loss over time. Perhaps future scientific research will show a correlation between loss of lean body mass on a diet with the development of redundant skin.
In the meanwhile, here's an example to illustrate how muscle loss on a diet can cause droopy folds of flabby, loose, skin. Let's say you start at a bodyweight of 115 pounds, in good toned shape with firm skin, but you gradually gain 100 pounds of body fat. You then go on some sort of crash weight-loss program or have bariatric surgery to quickly lose those 100 pounds.
However, let's say not all of the weight you lose is body fat. Perhaps 20% of your lost weight is muscle. This is not unusual on a severe crash diet, and many people lose much more muscle than that on these diets. In fact, if you were to fast on just water, 60% of your weight loss would be muscle. Besides, because muscle can rapidly dehydrate and sacrifice protein to provide an additional supply of energy, muscle loss increases the more active you are on a severe diet. But let's stick to a modest number of 20% muscle loss in this example.
Now, after your severe diet, you have returned to your starting bodyweight of 115 pounds, but your internal body composition is altered. You have 20 fewer pounds of muscle on your body, and 20 more pounds of body fat, even though you weigh as much as you did before you put on the extra weight! The percentage of your bodyweight that is fat is now higher than before you gained weight.
Would you expect your body to look as shapely and firm under these circumstances, with a higher percentage of body fat? NO! At 115 pounds, body parts that used to be filled with toned muscle are now filled with a greater volume of flabby fat instead. Pound for pound, fat takes up more volume than muscle, pushing farther out until it begins to hang in folds of skin. Continuing on with your diet only exacerbates the problem as your muscle level drops even more. Even worse, when your bodyweight dips below your normal starting point, you are now too thin and covered with flabby skin!
Now imagine the effect if you lose several hundred pounds of bodyweight, and your muscle loss is twice as much, 40-50% or more of lost bodyweight. Those voluminous folds of flab can become an enormous problem.
