I like to err on the side of having too much food when people come over. There's nothing worse than people looking around nervously, trying not to be impolite when they're still hungry. But, it's important to remember that people of different ages eat different amounts. So, what's barely enough for a group of hungry teenagers will be too much for group of middle-aged adults.
As people get older, they seem to become more conscious of the carbohydrates they're consuming, preferring to eat fruit to pastries. They also seem to be more cautious about managing their bladders, so they won't need to go to the restroom as frequently. When calibrating how much food to get for a party, don't forget to calibrate for how old people are and remember not to assume that everyone else will eat like you do.