No one spays their cat because they’re worried it’ll have half-dog-half-cat babies with their un-neutered dog (or that it will produce the monster that was CatDog). And it’s not necessarily a size thing. My cat is roughly the same size as lots of toy-sized dogs, but she’s in no danger of being impregnated by a Chihuahua. It’s a species thing. Dogs and cats are distinct species, incapable of producing offspring together because of the many differences in their genome. At least, that’s the traditional definition of a species, and it works—for the most part. It’s been many millions of years since whatever common ancestor dogs and cats share, and today they are so obviously separate that we make jokes about how different they are.