If nothing else, people tend to to put more effort into properly training large dogs out of any human-aggressive tendencies. It's a lot easier to view your Chihuahua nipping at someone's ankles as cute, compared to your pitbull doing the exact same thing.

Honestly, the biggest problem I've had with large dogs is poor socialization in how to play with humans. A dog doing properly-social play by dog standards (play-bows, carefully light nipping, etc) can still be a really worrying experience for an adult if the dog is 100lbs.