To try to provide another example, I spend a lot of time walking around on a college campus, and I'm far more likely to get run over by a student on a bicycle than by one of the semi trucks that stops by every so often. I got hit just the other day by a yahoo on a bike, in fact. However, if you asked whether I considered the bikes or the semis more dangerous, I'm going to answer "semi" every time. Why? Not because I'm more likely to get hit, which isn't true, but because if I do get hit by a truck, I'm going to be a grease spot on the street.
Same deal with the small dog/pit bull argument. You could show stats that say a toy poodle is 100 times more likely to bite than a pit bull, but virtually no one is going to die from a vicious toy poodle. A pit bull, on the other hand, is quite large and muscular enough to kill anyone younger than a teenager or anyone is handicapped in some other way against an attack.
In my book, this is what makes pit bulls and the like "a more dangerous breed".