I've had (and fostered) little dogs, medium sized dogs and big dogs. I've noticed no correlation between behavior and size. There are well trained and poorly trained dogs of all sizes.
I have noticed that some people have prejudices against certain sized dogs, and that prejudice often leads them to notice things rather selectively. It's really no different than (for example) people who like X brand of vehicles and people who like Y brand. If you like X brand then you tend to notice problems and issues with Y brand while ignoring any issues with your own brand. That's not the greatest analogy, but the best I can do right now.