While this is not necessarily true, I can see where the stereotype is coming from. In general, small dogs have been bred for companionship and looks, while most larger breed dogs are working dogs. So larger dogs are often more easily trained and willing to please, while smaller dogs are often more independent and a little selfish, to put it that way. While this does not mean they are dumb, they are just often more difficult to train, but they can still be smart. Depends on the dog.