This psychological effect, is known as social modeling. What it boils down to is that most people tend to eat less than they normally would if they dined alone when their companion consumes smaller portions of food.
As I was shopping online (and overspending like crazy, I might add), I happened upon an article on CNN Money. It had to do with budgeting and it included some good strategies about coming up with a budget and sticking to it. A handful included:
For many of us, the festive season around Christmas brings out our worst over-indulgent habits: we eat, drink and spend more than we would normally do, and regret it come New Year when we step on those scales, or the credit card bill lands on the doormat.
While it may not be desirable, or even realistic to aim for an indulgence-free Christmas, if you find yourself wishing that you could avoid some of the excesses, then perhaps consider having yourself a Mindful Merry Christmas.