Humans are generally more tolerant of undesirable bad behavior among small dogs. While same behavious on big dogs may be seen as potentially dangerous.

Small dogs may be overindulged and over protected, leaving their bad behavior reinforced.

Humans who think that a small dog acting like a tiny tirant is adorable just end up reinforcing that bad behavior. So a small dog that is petted when he snarls at a child learns that this aggression is wanted and ends up reinforcing these actions.

Here are some translation facts from dog behavior to action

Barking
A small dog who is anxious or insecure may bark, lunge or growl at other dogs passing by on the streets.

Begging for food
While many dogs will beg for scraps a canine with small dog syndrome will be particularly persistent.

A disregard for rules
This could manifest as a dog who refuses to be house trained, or a dog that will not listen when he is told to get off the coffee table. He is the boss, and he rules his kingdom.

Persistent whining
The small dog who whine when they don’t get what they want could have an attitude problem.
Yet the strategy works because the household often revolves around keeping the dog content and quiet.

Resource guarding
She chases other dogs away from toys, food and water bowl. If you’re still on the couch or at the park she growls at another dog who is trying to say hello to you.