In my experience the people who gain lots of weight over the holidays are usually people who are already overweight and/or don't pay attention to their intake. We have multiple family/friend gatherings over the course of a few weeks with large amounts of high-calorie food, and it's easy for my family members who don't pay attention to their intake to pack on the pounds. Most of my family is fat (and stay fat) because they just accept it and don't do anything to counter that extra intake.