The bigger your dinner group, the more you are likely to eat.

People eat when they're not hungry. It doesn't really make sense, but we do it all the time. It's not healthy for you. Television commercials have the power of suggestion to get you to eat when you're not even hungry. People also eat for social purposes, regardless of how hungry they are. Eating in a big group with 8 friends? You're likely to consume TWICE as many calories!
(source)