Size difference is an issue when it comes to dog interaction. Big dogs need to learn how to be calm around small dogs and small dogs need to be confident around big dogs. Small dogs often bark at big dogs most often because they would like more space and are intimidated by the big dog. Big dogs often trample or roll little dogs, most times accidentally. Everyone learns from accidents, but most likely, not what you would like the dog to learn. Big dogs may learn that chasing small dogs is fun. Small dogs are reinforced that they have a reason to be scared of big dogs.
Teaching young, large breed, puppies to lay down during an interaction with a small dog is an easy way to set expectations early and reinforce wanted behaviors throughout the dogs life.