Or, the less mean dogs were lucky that humans only killed the threatening ones?
Luck's got little to do with it. Dogs were selectively bred for those traits.
Little dogs are mean because shitty humans think “aw he’s so itsy bitsy I don’t have to train or discipline him at all!” Totally ignoring that (a) a dog needs maintenance discipline to be emotionally secure, and (b) aggressive little dogs are unhappy little dogs.
My mom and sister have chihuahuas and my grandfather has a shih tzu, they are all super sweet dogs who have ever been aggressive towards anyone. Meanwhile, a family that lives down the street from my grandfather had a golden retriever that had to be put down because it kept attacking kids unprovoked. Sure, some dogs are more predisposed to be aggressive than others but the way they are raised and the environment they live in plays a massive role in how a dog acts.
Are bigger dogs less mean, though?
From anecdotal evidence. Little to medium stray dogs are the ones most likely to snap and chase you for whatever reason. Granted, it could just mean that big mean dogs were put down ebcause they actually hurt someone compared to the abandoned dog with a napoleon complex.
But dogs started out relatively big and we made the smaller ones after they'd already been domesticated
