The phrase "tallest dog" refers to a dog that is taller in size compared to other dogs. It means that this particular dog is bigger in height. Full definition
The world record for tallest dog is currently held by a Great Dane. (wideopenpets.com)
Some breeds of dogs are taller than others, but have you ever wondered which one holds the record for the world's tallest dog ever? (dogs.lovetoknow.com)
They have thick necks, shiny black coats, and are tall dogs who weigh up to 130 pounds. (pethelpful.com)