Rapid weight loss will result in excess folds of skin. Your skin needs time to shrink back to it's smaller size. Skin is very elastic and will tighten slowly as you lose weight - SLOWLY. To help skin tighten as you lose weight you will have to exercise to tighten the muscles underlying the skin.
If you choose to diet without exercise then plan on visiting the plastic surgeon after your weight loss for some expensive nipping and tucking.