Just my 2 cents, but the two Chihuahuas we own are far more aggressive than the Rottweiler, Basset hound, or Lab Mix we own. In fact they are more aggressive than any other dog we have owned or fostered, which include rescued rottweilers, pit bulls, labs, retrievers, and just about any mix of large bred dog you can imagine. In my opinion it has more to do with personality and training of the dog. Our little dogs know they are little and because and act aggressive when they know that the big dogs are behind them, in fact when they look back and see that the Rotti has lost interest they decide that whatever they were barking at is not worth barking at. They are small and they know it, and we didn't work as hard to make sure they were trained properly and because of it they think they can get away with thinking they are larger than they are and this is our fault. Every time we own or foster a dog now we make sure to train them in how to act with people, it doesn't matter the bred.