Why do overweight/obese people gain weight so fast and get even fatter after being on a very restrictive diet?

Insulin resistant: Insulin is the hormone that drives sugar into the cells where it is stored as fat. Most obese folks are insulin resistant, and have high levels of insulin. That's why Metformin is such an effective weight loss agent. Metformin restores insulin receptor sensitivity, reduces the body's insulin requirement, resulting in reduced carbohydrate cravings and reduced storage of fat.