I've only had to care for 3, each at different times. Each was the same though, elderly lady who lived by themselves and their dog. The dog only gets attached to that one person, and then when that person either dies or can't care for the dog anymore, the dog is now put in a completely new environment. My only guess is that they are distrusting of new people because it's not who they're familiar with. I can guess that could happen with any size dog however. By the same token, I also realize that there can be small dogs socialized very well and don't have a propensity for aggression. I just find it ironic the only aggressive dogs I've usually had to deal with were small enough that I could kick them to the county line IF I wanted to LOL
The phase "little man syndrome" does come to mind though to apply it to dogs LOL
To me, an aggressive dog is bad, no matter what size they are.