People aren't fat because they eat too much. People (and mammals) eat too much because their fat cells are being metabolically triggered by environmental and/or genetic factors to store fat at a higher than normal rate, diverting calories from operational needs, thus stimulating hunger.
Bears don't go into hibernation because they've gotten fat. They get fat because their endocrine system begins gearing up for hibernation, and telling their fat cells to kick into overdrive growing. They will literally burn muscle while simultaneously gaining fat in this stage, like all hibernating mammals. The other side effect of growing fat cells is massive hunger, obviously.
In humans, simple things like not eating sugar can help correct the endocrine factors which, for a lot of people, cause the fat cells to do this. However, genetic factors are going to be a different story.
I say this as somebody who is fit and active, but have had running buddies who are fat. Pitting will-power against metabotically driven hunger ends, in the long-term, with biology winning every time.
Without endocrine/genetic factors taken into account, reduction in calories in simply causes a mammalian body's system to reduce calories out by making the subject less energetic.
