Honestly... I don't know.

In my opinion, I see just as many irresponsible owners of large dogs as I do little dogs. It could be that people with little dogs just tend to take their dogs on outings, where they are going to meet people and other dogs, compared to larger breeds. It could be that, were a Mastiff behaving the same as that dog, the owner would be physically unable to control him, hence that untrained Mastiff never seeing beyond his backyard.

I think that uneducated people aren't really biased between breeds. If some one is irresponsible enough to not bother to train their little dog, then, if they had a large dog they probably wouldn't either.