For some obese people, cutting food intake merely hikes up the amount of food being digested. To be indelicate, instead of a lot of food passing out as turds, bowel movements diminish drastically because almost all is being digested.
Worse, sometimes dramatic cuts in food intake can lower blood sugar to the point of weakness, yet stored body fat is still untouched. The problem seems to be not just that obese people have to eat a lot to support the weight, and if they don't their blood sugar drops too low, before fat reserves can be converted. It seems to be the case that fat cells may not give up the reserves till the equivalent of starvation sets in.
Dieting is one thing, but practically starving yourself is another. I'm not so sure we can redcue this to a simple lack of will power. Perhaps the people whose fat cells are less stubborn are more fortunate than strong-willed?
I've read that the body can even start absorbing protein from muscle before it can access some fat reserves, particularly the kind called "brown fat." This seems rather extreme and I don't know if this is reliably confirmed.
One early report I read said that obese people often had a more varied suite of intestinal bacteria. Since these bacteria play a role in digestion, the implication is that the food they ate was more efficiently digested, providing a surplus of energy than others with a more impoverished intestinal flora. The result is that the same, normal seeming diet results in weight gain. It's not really a secret that weight gain has a ratchet effect, where it's harder to lose than to gain. How much harder, again, seems to have more to do wtih physiological factors than will power.
There is a very high noise-to-signal ratio in nutrition, which historically has been a paradise of cranks and cultists. As such, it is remarkably easy to incorporate social prejudices about personal responsibility, which usually means an imputation of personal inferiority. I think it would be wiser to be cautious. Is weight gain while aging truly a sign of degeneration of character? Or is it a side effect of slowing metabolism?