A couple of thoughts: is the little dog crying out in pain? Sometimes, what looks like serious aggression actually sounds a lot worse than it is?
Then the other side: my daughter had a spayed female pointer mix ("Daisy") - and she was truly possession-aggressive and would draw blood. And give absolutely NO warning that she was going to attack. And her "prized possessions" could be any ridiculous thing: a dead housefly, for example. Plus, she was just generally "bossy" around my neutered male dogs.
My daughter had relocated back to this area, so Daisy was living with us, too. After she drew blood on my Cattle Dog (a 1/2" gash just below his eye) - I knew I had to lay the law down. I set her up with some really good possessions and my shock collar set on "Nuclear Holocaust". Then waited. For over an hour - until she turned on the Cattle Dog (he walked within 5' of a prize) and I let her have it. The idea is that if she thought my dogs were SO ferocious that they could inflict such pain on her in the blink of an eye - she'd leave them alone. It worked - I more or less rearranged the Pecking Order to put her at the bottom. But doing that requires diligence, and PERFECT timing.