Most species of animal are about the same size. Except for dogs. Dogs have the greatest size variation of any species of animal. While all dogs are descendants of wolves, they have been selectively bred for thousands of years for different purposes.
It is surprising that given the variability among dogs they recognize other dogs. For example, Oliver, the 150 lb Bernese Mtn Dog that lives across the street knew right away that Lilly, the new 4 lb Dachshund puppy from down the street, was another dog when they first met.
A recent experiment confirms that dogs can recognize other dogs by sight alone. In the experiment dogs were presented with two pictures from a distance – one of an unfamiliar dog and one of another animal such as a cat, sheep, gerbil, cow, rabbit, reptile or bird.
The dogs were rewarded if they approached the picture of the dog rather than the picture of the other animal. The dogs in the experiment were all able to consistently select the dog picture from a distance, even though the pictures of were of unfamiliar dogs of varying breeds and from varying angles.
The source for this IFOD and details of the experiment can be found here: https://blogs.scientificamerican.com/dog-spies/do-dogs-know-other-dogs-are-dogs/