Thank you for all the suggestions. Social eating issues come up frequently on3 Fat Chicks, so I know I am not alone. I do take a green salad or a cooked vegetable salad for some lighter fare.
I guess I should have said that I was too full to eat that big of a slice, cut off a couple of bites and return the rest to the tin. I have a thin neighbor who likes dessert (obviously not too often

) that I could pass stuff on to if I took some home because "I was too full". That is certainly true enough only 1-2 hours after a meal. I am trying to learn to eat only when hungry.
We meet at noon on a Saturday, so it's lunch. That actually makes it easier to compensate some later. I basically only need a snack before bed. One dessert would certainly be plenty for our group, but having a lot of dessert seems to be an important part of the get-together.