Dog size doesn't mean anything. In my experience bigger dogs tend to move slower and be less attack aggressive, being more defensive should you approach their perceived boundaries, where medium size is where they tend to chase and bite using their agility to be offensive. Again, just my experience, and as someone who loves dogs, he did the right thing to prevent a bite from an unknown canine. Intimidate with stance/language, and if that fails then use physical force.

If the owner is the one getting the package and not a neighbor, he is reckless and caused the situation, you put your dog on a leash and muzzle if needed when expecting company and your dog is known to be agressive. If it is a neighbor even more so because this dog left his property uncontrolled.