Because that’s what happens when you eat ten full plates of turkey and pie. However, Bonnie Berkowitz and Lazaro Gamio for the Washington Post go into some of the specifics of holiday eating. The last bit of the feelings section (part of it shown above) on permanently heavy:

After the first 750 calories or so, your body begins to store a larger percentage of food as fat. A 2000 study found that the average adult gains a pound during each holiday season and usually never loses it.

Wait — really?