Skin is an amazing protective layer that covers the body. As you gain weight, your skin expands, stretching to accommodate additional fat. We often compare the skin to a plastic bag. If you fill the bag past its capacity, it will stretch and expand to accommodate the additional contents. Remove the items inside, and the bag will have likely lost its shape. It’s still a bag, and it still holds things, but it may be stretched out in some areas and lose its sleek appearance.
The skin has some ability to contract, but in cases of massive weight loss, the amount of extra skin exceeds the body’s ability for contraction. Exercise can build muscle, but it won’t tighten loose skin. Cutting calories burns fat, but the skin will still remain.