Given the incredibly low odds of the obese returning to a normal weight (especially the morbidly obese), I think it's time we stop looking at body weight as a function of willpower.
The simple fact is, you just cannot delete all the extra fat cells. Once obesity is established, a new set point will be established, and the brain will do everything that it can to return the individual back to the their set-point if they try to deviate too far below it. In fact, putting an obese person on a diet will eventually elicit a response that mirrors starvation in non-obese people, well before that obese person attains a normal weight. This is especially true of the morbidly obese.
The simple fact is this: fat cells, when shrunken, are much less efficient at releasing leptin, which means that when you compare two people of the same fat mass, but one has lost weight to get there, the one who has lost weight will have less circulating leptin. Leptin is the hormone that signals long-term nutritional status and is released by fat cells, where it circulates in the blood and crosses the blood-brain barrier so it can signal to a special area of the hypothalamus that you're fed. When the leptin signal becomes weak, the body responds by influencing a number of downstream factors that cause a significant reduction in energy expenditure and an increase in appetite. This increase in appetite can be measured across several domains, including physical hunger, food seeking and the response visual food stimuli. Bottom line: a person's normal weight is controlled by powerful homeostatic mechanisms that are almost invariably impossible to overcome in the long-run without drastric interventions. At best, an obese person can expect to obtain a weight loss on the order of 5%, which, fortunately, can substantially improve metabolic health, assuming they can maintain that 5% weight loss, along with a healthy lifestyle.
If you have the time, you should watch the following video by Jeffrey Friedman, who is one of the top researchers in the field of obesity and is the co-discoverer of leptin, for which he has won many prestigious awards.
https://www.youtube.com/watch?v=5pBj0wy35A8