Small dogs tend to be a bit more lonely than large dogs so they usually bark more often than the larger dogs to get attention.
Also some small dogs are not really taught proper behavior and are sometimes seen as cute by their owners when they bark at things such as for food etc which is wrong.
My aunt has a small dog that whenever she gets a plate of food that dog will immediately be at her feet or on her lap barking at her until she gives it some food.
She thinks it's cute that the dog does that but it really is not and is bad behavior.
Dogs should not be barking at people begging for food as that is just plain rude and needs corrected.
Smaller dogs will also bark more at things because they're smaller than other dogs or animals and are barking to defend off those other dogs, animals or other things that may be coming near it.
I also find that smaller dogs have a more annoying piercing barking sound than large dogs do and I can't stand when any dog constantly barks but small dogs barking is the most annoying.