I don't know how to answer your question but I have a horrible big dog/little dog story that I'm really embarrassed to share for fear of being called an irresponsible dog owner (and maybe I was), not to mention that even today I find it almost too painful to tell.
My small beagle is my oldest dog and older than my next oldest dog, a female GSD, by 4 years.
This story goes back to 2004. We live in a rural area, and have a very long driveway. Since I have insomnia and since that particular night there was supposed to be a meteor shower, I went with my then 4 dogs for a walk down that driveway. It was a perfect night for star-gazing - so dark, I could not even see my hand in front of my face.
It was also not unusual for me to take these walks late at night with the dogs.
I was maybe a quarter of the way down our 500 metre driveway when I almost tripped over the beagle who I could not see. I said to the beagle, "Careful," and the next thing I knew all hell broke loose.
My female GSD attacked the beagle. There were no growls, no barks, no warning. I was rolling around with the dogs in the ditch, trying to separate them but my female GSD was in kill mode without a doubt. I finally found her head end in the darkness but she would not let go of the beagle. She had her by the throat the way a cat will kill a mouse.
I was screaming at the top of my lungs to make my female GSD let go. My husband was sound asleep in the house some distance away and there was no one that was going to hear me in the middle of nowhere. I finally stuck my hand in my GSD's mouth because stupidly or not, I was convinced she would never bite me, and that bought the beagle a second of time as the GSD spit my hand out and then went at the beagle from a new angle.
I shut up with the screaming when I realized that my GSD was taking it as encouragement, and kept struggling with her by putting my hand in her mouth. This may go against all common sense and I wouldn't recommend it to anyone but she didn't bite me.
The beagle was maybe 25 pounds. The GSD would have been about 90 pounds. Her collar came off in my hands. I had had previous arguments with my husband because he seems to think a dog collar should fit like a ladies' necklace - loose - and he had loosened the collar unknown to me.
Even after I was sure that my beagle was dead because I didn't think there was any way she could survive such a sustained attack, I kept trying to get my GSD to let her go.
Out of nowhere, vehicle lights appeared, thereby illuminating the beagle, completely limp, in my GSD's jaws. It turned out that my husband thought he had heard something through the opened bedroom window, and once he realized I was not inside, came looking for me in the van.
With the lights my GSD dropped the beagle and went bounding off to the van, ready to go for a ride.
Luba was lying completely still where she had been dropped. I called her name and her tail went up once in acknowledgement, and then down. I grabbed her and ran to house.
My vet met us at the clinic. Luba's voice box had been crushed and she was near death but not dead. She had surgery. My vet has been our friend for many years and he allows me to stay with my dogs if I want to. I stayed. I can't remember but I think two days' later she was allowed to go home - my vet later told me to die at home. He gave me syringes and painkillers etc, to keep her comfortable.
She was home 2 days, and then her head swelled up so much she had to go in for another surgery. Her blood vessels had been so crushed that the blood that was pumped normally to her head could not drain. She survived the second surgery.
When she came home again, I slept with her on the floor. And naturally, I kept the dogs separated because every time my GSD saw her, she wanted to finish the job.
And I admit I took a good long look at my GSD. She had been socialized from puppy-hood, exposed to all kinds of situations and people. Kids liked to try and ride her like a horse and she'd never shown any signs of aggression.
It is only in the last 3 or 4 years years that I've felt confident enough to leave the two of them alone together. My vet says that female dogs are far more aggressive with each other than male dogs are, or males and females are. Just like people.
I agree with the poster who said that little dogs provoke the prey drive in larger dogs. But what I also think is that because my GSD is so totally my dog, she somehow perceived a threat when I was almost tripped by Luba. And I also have seen how a little dog can deliberately provoke a larger dog. Luba does it to the GSD all the time.
I think I used to see some of Luba's behaviour as almost a joke - the little dog challenging the bigger dog, trying to steal the bigger dog's food, marching face to face with the GSD, her tail wagging straight up and obnoxiously, her entire body language just daring the GSD to make something out of it. I think that is the problem with small dogs - that what people would see as a problem if they were bigger, they see as 'cute' in a small dog when it is not cute. I have learned my lesson there.
I now put a stop to it immediately. I think I stepped out of the leader's position and the GSD thought she had to defend herself. It took time to get there, and included a period when my GSD had a muzzle as well as a leash on in the presence of Luba but now all I do is tap Luba when she goes into her attempts to provoke the GSD, and it seems to be enough for the GSD that I know what is going on and put a stop to it.
With hindsight being 20/20, there has never been a time when my GSD has provoked Luba.
That doesn't mean that trust didn't have to be rebuilt between me and my GSD - it did - not from her end but from my end as to how well I thought I knew my dog. Although we don't have children, we have many children who come here and the last thing I want is a dog I can't trust.
I trust my GSD with children and even with Luba to a certain extent, but I would never leave the two of them alone together for an extended amount of time. My GSD seems to bear no more animosity towards Luba (the reverse is not true as every now and then Luba seems determined to annoy the GSD with a kind of 'don't think I don't know what you did to me, you big bully" attitude. But they can coexist.
I can't say that I would have had my GSD euthanized if she had continued to exhibit aggression towards Luba since I do have the ability to keep them apart and safe.
I agree with Samara that dogs don't think in terms of the mismatch in size, even though, yes, big dogs can cause a lot more damage.
Luba was so close to death and her recovery so miraculous in the eyes of my vet, that every time I take Luba in for her shots, the entire office comes to a standstill while the vet regales his staff with the extent of her injuries, her two surgeries, and how he never thought she'd make it.
It was the first and only serious fight I have ever seen between dogs and it isn't an experience I would wish on anyone. As for the 3 strikes rule, I think rules depend on what happened, including who started it. One size rules can be unfair. I don't absolve little dogs for their actions.
And I do feel that I underestimated the power and the single-minded focus of a GSD.
Even though that was not the case in my case, I agree wholeheartedly with you that dog owners ought to keep their pets at home/under control and that the onus, fair or not, is more on the person who has the large dogs.
I'm so embarrassed.