I tend to lean towards “nurture” in the debate of small dogs being more aggressive than larger ones.
Too many people get small dogs and then try to treat them as if they were either toys or big dogs. Both approaches are wrong for dogs whose adult size will be less than 10 pounds.
Little dogs are aggressive because they are afraid. The world is huge and they know they are small and fragile. Aggression is a camouflage for their fear, a display to intimidate that which scares them.
Therefore, training needs to be geared towards teaching them they are safe, and giving them the tools they need to feel safe.
These small dogs need to be trained where the human trainer is near their height level. That means either the human trains while lying down, or the puppies are elevated and trained atop a desk or table or counter (with safeguards so they don’t fall off). After they learn at a decent level, they can be lowered until they are learning from the floor, but their initial training needs to be at easy eye contact level.
Eye contact in training is important and tiny dogs are at a disadvantage because of their size and distance from the human’s face. Eye contact lets them feel they are communicating. Communication helps them feel safe and builds confidence in the tiny dog.
Slower movements in training is also essential. The puppies need warning when movement will happen until they learn to anticipate it and that it’s not going to hurt them. Repeating the movement in slow motion lets them learn the movement is a safe one. Slowly speeding it up teaches them confidence that this movement won’t hurt them. It reassures their fears and helps them feel safe.
A softer, slower tone of voice also inspires confidence in the tiny dog and teaches it to be calmer and quieter, too. This is another aspect of feeling safe.
Teaching the tiny dog hand signals and sign language, and responding to the dog’s signals builds confidence. It’s part of the communication between human and dog, and it helps the tiny dog feel safe.
Providing the tiny dog with a safe place that it can retreat to and know it will be left alone also helps eliminate aggressive responses. Tiny dogs love cave beds and crates because they feel safe in them. In agility training, dogs are given a pause box or pause table, and it helps to have one of those for tiny dogs. They learn they are safe on that spot and by using a pause spot for giving treats and teaching new tricks, the tiny dogs learn to love that spot.
Tiny dogs also need a job. They aren’t just ornamental. They often have higher energy levels, and need a way to use that energy so it doesn’t become nervous or aggressive energy. Jobs that tiny dogs excel at are things like alerting to timers, picking up small objects and putting them in a basket, doing certain tasks or tricks when a timer goes off, separate recycling, put laundry in a basket, open and close curtains, close drawers and cabinet doors, turn lights on and off, giving back massages, and other small tasks. Some of these tasks the dog can do while you are gone, and others the dog can do for you.
My service dog has all the work he can handle, and staying focused on his tasks helps him feel useful and increases his feelings of safety and confidence.
The two pet dogs have been taught a series of tasks that they can do throughout the day – picking up toys and seeking hidden treats, opening and closing curtains, filing their nails on their scratching board, getting their lunch when the timer goes off, lining up at the door when I get home, that sort of thing.
This keeps them busy all day, they don’t have much time to get bored, and they know when I’m coming home and what to do. Knowing what is going to happen next keeps them confident, and helps them feel safe and useful.
Remove fear and tiny dogs can be as calm and well behaved as larger dogs.
I sincerely believe nurture has far more to do with aggression in tiny dogs than nature.