The problem is the metabolism doesn't reestablish its homeostatic baseline.
For example, after forcing myself to gain 35lbs over 18 months, I started losing
fat without any effort.
Likewise, after restricting food for a few months and dropping to 7% body fat, I gained weight when going back to eating ad lib.
This is how the metabolism is supposed to work.
Your metabolism is designed to keep your body fat in a healthy range. It's not supposed to allow your body to continue to pile on fat
year-after-year leading to obesity, diabetes, and the sequalae of disease that results from metabolic dysfunction.
So why don't the pounds just fall off after the over-eating holiday season?
The kind of food that is over-consumed determines how well the metabolism works.
Junk food, full of vegetable oils (high in omega-6 polyunsaturated fats), impairs proper metabolic functioning.
This is why, if I were forced to choose, I'd choose sugar over seed oils. Though most junk food combines the two.