Small dogs can be aggressive also. Sometimes more aggressive because of anxiety:

http://www.oes.org/page2/332~Small_Dog_Aggression.html
http://dogs.lovetoknow.com/wiki/Anxiety_in_Small_Dogs

Even books have been written. If you take the time to read the reviews you will see that there is info on why small dogs are aggressive and how to handle it. From a review:
"It also has many solutions to the dogs aggression toward people, children, animals, and other dogs. All of the examples of things that people should do while training their small dog, helped me understand why I need to do different things while training."

http://www.amazon.com/Living-Small-Toy-Dogs-Personality/dp/0931866790

Let me know if you need more.