I had a Dachsund once that could put any big dog to shame - he was afraid of nothing, even tried to chase horses. He once chased two St. Bernards down the road. Thing is, he was little, so you could just grab him and pick him up if he was bad, so every one thought his badness was cute.
I have a Blue Heeler now that everyone loves, but she's sent two other dogs to the vet for stitches (both were mine, food related stuff, I now watch her like a hawk). She's very dominant and I would trust a Rottie over her. I was asked to take her by the humane society that I sometimes work with, as she was too much for a regular home. So, it's not just these bigger "dangerous" breeds that have the potential to do harm. In fact, any dog can and will bite if scared or irritated enough.
I maintain, however, that cats are worse and the only reason they don't eat us is that they're too small. :) (Have 4 cats, love em all.)