I gained five pounds in Europe. And it was optimal. Partly, that's just because the food was both good and different from what I can easily get in Virginia. The fundamental reason I gain weight on vacations, though, is that hunger is a greater risk when I'm away from my home base.
During my average day in Fairfax, I never worry about being hungry. If I get hungry, I know a dozen easy ways to solve my problem. On vacation, things aren't so easy. The restaurants may be closed just when my tummy starts to grumble. I might be in the middle of nowhere. The people I'm with may not be hungry when I am. Etc. Filling up when food happens to be available is a good insurance policy.
The flip side of this lesson, though, is that there's little excuse for stuffing yourself when you're at home. You can safely err on the side of under-eating, because hunger is so easily sated.
As I've explained earlier, my fundamental dietary strategy is never to eat unless I'm hungry. On vacations, that strategy is a little too risky for my taste. But during the rest of the year, there's very little downside to moderation.