I think your title hits the nail on the head better than talking about training. It may be that people who do not have the time, patience, or access to formal training are more likely to get smaller dogs, because there is less chance that the dog will cause any real damage to another's person or property. As long as the person is willing to control their dog in all situations it doesn't really matter if they have formal training. It may mean that the people with smaller dogs have to physically contain their dog rather than controlling by voice or habit. That is fine as long as you control your dog. I personally find it easier to teach the dog manners than to always be on alert to the dog's actions. It is never alright to be an irresponsible pet owner no matter what size or species of the pet.