Compare
How big is it really?
Cat
21 mm
Dog
20 mm
20 mmA Cat eye and a Dog eye are about the same size.
Some of these are measured front to back rather than across.
Pupil shape
Cat: a vertical slit pupil
Dog: a round pupil
Vertical slits belong to ambush hunters close to the ground. They help judge the distance to something before pouncing. Round pupils are most common in animals that hunt in the daytime and go looking for food. Tall hunters have them too, and so does almost every bird.