In the vast majority of cases, dogs are bigger than cats. That knocks me out. Cats are felines, which means that they're related to tigers, which are Jeep-sized juggernauts that could kill you with one swipe of thier paw. A domestic cat, on the other hand, is spry but only about the size of a human baby.
A Great Dane, on the other hand, is about as big as any canine that's been on the Earth since the last ice age.
Ferrets are more straightforward. There are no tiger-sized mustellidae. It would be weird if there were.