I have to say, I have done rescue work for years. THe county I grew up in did not have a shelter, so strays and 'unwanteds' were brought to the Vet, who was my neighbor. We fostered for years, and when I moved out of the house, I continued the family tradition of foster and rescue work. He knew that I was used to large dogs and "dangerous" breed dogs, so that is usually what he'd call me for. I have taken in some seriously messed up (physically and emotionally) pits, rotts, shepherds, etc. etc. and had to do some pretty brutal evaluations as to whether or not the animal could be rehabilitated enough to a "forever" home.
I LOVE the large breeds, pitts, rots, mastiffs, danes, shepherds, etc. I think that they are WONDERFUL animals, and can be a wonderful family dog. I do not buy into the belief that certain breeds are inherently "bad", but, much like people, many factors go in to determining what kind of personality a critter or person is going to grow up with. The environment that the parents are raised in is huge. The environment that the puppies grow up in during their first weeks of life are CRITICAL in the development of the personality, temprament, and disposition of the dog. That is why I ALWAYS recommend that families buy puppies where they can see the litter, meet the parents, etc.
I have been bitten severely only 3 times. Twice by dogs under 5 pounds, and once by my beloved Shepherd, when she was losing her grip on reality. (she had been horrendously abused, and she was the one exception I made in my "can this dog be rehabilitated" evaluation, and I almost paid for it with my life.)
Yes, small dogs are naturally more 'fearful' because they are small, but I agree that it does NOT give them license to bite, snap, snarl, etc.
How a dog is raised, how it is trianed and how it is socialized makes a huge difference.
I had a 5 lb rat terrier/jack russell terrier mix as a rescue. I got it at 5 weeks old, almost starved to death, and fit in the palm of my hand. dropper-fed it back to health. Took that little bit of fluff to the dog park every day to play with dogs big and small, and it never knew it only weighed 5 pounds. His favorite wrestling companion was a 165 lb mastiff, which was funny as heck. He didn't growl, he didn't snap. If he thought he was going to get stepped on, he'd give a little "yip!" sound and jump out of the way.
I think ALL dogs and all owners should go through obedience courses. It makes life much easier for everyone.
My 5 cents.