Cats are mammals; the difference between ape and feline tits is basically, the smaller you are, the more babies you can have. Large (over 100 lbs adult weight) mammals have fewer babies than smaller ones. Bears and elephants have two chest-tits, too. Tits tend to match how many kids you have at one time.

And frankly, what I understand of the Khajiit is that their (fantastical!) biology is REALLY weird, in that how they develop depends on the phases of the moons. Yes, those tiger and kitty-like Khajiit in TESO are people, too, and are perfectly understandable to the more humanlike (playable) Khajiit. When that one dude refers to the tiger as his brother, he's NOT being metaphorical.