the temperment also has much to do with the owner and the way the dog has been raised. The fact of the matter is that if a large dog isn't trained well, then you have a large problem. however think of how many smaller dogs are trained so poorly, but because they are small its a small problem.
I've seen more vicious poodles and shitzu's than I have seen viscious rotties. The difference is, most people aren't afraid of a small poor tempered dog. Personally, when I had my rotties, I used the 'kohler' method for training. Its a really hard thing to do, but it was the best training I could give my dog. It works, and I never had to worry about her misbehaving.
note: imagine if poodles had the same power as a rottie? You'd be reading about their destructive behaviour all the time. (most poodle owners I have met let their dogs do whatever it wants.)