Little dogs tend to be more prone to "get them before they get me" - it's got to be a pretty scary world from their perspective because EVERYTHING and EVERYONE is so much bigger than them.....then add in the way most people tend to interact with a small dog, the tendency for people to let a lot of really bad behavior slide from a small dog under the perception of it somehow being cute/harmless, people who think that small dogs don't require the same training as large dogs, the fact that a lot of folks choose small breeds for situations in which they are entirely ill-suited - just a perfect storm of situations that end up with smaller breeds being almost setup for failure.