Carnivores tend to sleep longer than omnivores, who tend to sleep longer than herbivores. For a hunting carnivore, energy comes in big bursts, so it makes sense that they would be active for a short period of time, and hoard energy when they didn't need to be active. For a cud-chewing herbivore, time spent not chewing is time spent not creating energy. Obviously, this is a broad generalization - feeding habits, day/night cycles, predator/prey behaviors all factor into a particular animal. But it probably explains why your cat, like the panther at the zoo, spends most of its time asleep.