The breed differences are more exaggerated in dogs than any other common pet species. Yes, there are 2-pound rabbits, and 20-pound rabbits, but there are no 180+pound rabbits. The same goes for cats, gerbils, hamsters - there aren't any 100-pound pet cats around, or guinea pigs the size of a beagle, never mind a GSD!

Everybody knows that there are ponies and horses, but most are confounded when they meet a miniature horse, or simply view them as an oddity. I can think of no other species of pet that has the range of difference in size, weight, height, coat and temperament as dogs do.