If you can carry on a normal conversation while walking, that is to say, you're not trying to catch your breath, then you're not exerting enough energy to truly burn off the extra calories you are eating.
I can join the club of leaving Disney World with my shorts feeling tighter! Vacation to me is an excuse to eat whatever I want, so it doesn't bother me! Just letting you know I feel your pain!
I usually end up gaining a bit but I eat WAY more there than I do at home. It's those big portions at counter service and the sit-down meals that serve bread and salad and big entree and often I get dessert as well!
Having said that, I don't mind much. Like another poster said, it's vacation! The variety of cuisine in my little town makes Disney seem like the restaurant capital of the world!