I did find some interesting discussion of weight gain though:

From the results section of the abstract: As compared with their weight in late September or early October, the study subjects had an average net weight gain of 0.48±2.22 kg in late February or March (P=0.003). Between February or March and the next September or early October, there was no significant additional change in weight (gain, 0.21 kg±2.3 kg; P=0.13) for the 165 participants who returned for follow-up.

From the conclusions section of the abstract:
The average holiday weight gain is less than commonly asserted. Since this gain is not reversed during the spring or summer months, the net 0.48-kg weight gain in the fall and winter probably contributes to the increase in body weight that frequently occurs during adulthood. [Link.]

I might have gone wrong somewhere, but it looks to me like the average weight gain in the NEJM study is roughly 1.5 pounds per year (combining the weight gain from Sep/Oct to Feb/Mar and the subsequent weight gain from Feb/Mar to the next Sep/Oct gives me 0.48+0.21Kg, which I think works out at just about 1.5 pounds). The researchers actually refer to news reports (e.g. CNN) that claim average weight gain of 7-10 pounds over the holiday period, but their results suggest that these claims are some way from the truth.