How to stop larger dogs from packing up on a smaller one?

Whenever you've got a group of dogs together (especially when they aren't very well acquainted with each other) supervision is imperative. If I ever see dogs ganging up on one of the group (regardless of size) and they aren't behaving dangerously, I'm focusing most of my attention on the target dog. Are they responding appropriately, such as giving peace/appeasement gestures, like turning their head away, lip licking, yawning, or removing themselves from the situation? Are they giving fair corrections to the offending dogs? Or are they showing signs of stress/fear and overreacting to the rude behavior of the other dogs (appeasement gestures, a flash of teeth, air snap, warning bark or growl may be appropriate responses to rudeness, whereas snarling, hard stares, biting, or excessive barking/growling is a sign they are feeling out of control of the situation)?

Depending on what I see from the target dog, I'll respond differently to the offenders. If the target dog is confident and behaving appropriately, I'll step in to distract and redirect the offenders, basically backing up the target dog and supporting their appropriate behavior. If the target dog is fearful or overwhelmed, in addition to redirecting the offenders, I'll help remove them from the situation, and in the future I'll consider separating the dogs for playtime to prevent the situation from happening in the first place. This allows the dogs to have their fun but protects the fearful dog from being placed in a stressful situation in the first place, which builds trust between you and can prevent injury.

A spray bottle filled with water! Just squirt the guilty dog whenever he/she starts ganging up on the smaller dog(s) while saying a firm "NO"....make sure the nozzle is set to a full blast, not a soft mist. After a few times of having to do this, the dog will catch on if you just grab the bottle in their presence.