There's a reason why "heavy appetizers" are named such — when serving heartier appetizers in place of a meal, each item is meant to pack a filling punch. Holiday cocktail party apps tend to be packed with calories, too, but I have a trick to prevent overeating at these functions.
We all know that it's best not to arrive on an empty stomach; that will make you absolutely ravenous for anything in sight. After munching on a healthy snack before you leave the house, employ this strategy when you arrive: If there are passed appetizer platters at the party, decline the first time you're offered each type of hors d'oeuvres. Chances are all the appetizers will make a second appearance later in the evening.
This will allow you to check out your food options so you're not wasting your calories on something that you don't really want. You'll be more satisfied knowing that you indulged on food that you truly enjoy and didn't just jump at the first thing presented to you. On top of that, you'll likely consume less calories throughout the night.