Healthy eating comes naturally to you. You're sporty, love the great outdoors and lead a healthy lifestyle. You don't eat healthily just to be thin, but you don't like feeling heavy - it's just not you!
You can watch your friends stuff down huge platefuls of food, but you just have your fill and eat enough to stop you getting hungry a few hours later. It doesn't take much to fill you up, which is a definite advantage post-Christmas. For a couple of days (1 week max), just reduce your portions without being selective about what you eat: you don't want to deprive yourself of the nutrients you need. Listen to your body and your appetite - they're the best indicators you're on the right track.