My adult cat Joey has no problem chasing larger cats out of the yard or even backing off a raccoon, but is terrified of kittens.
We do kitten foster. Twice now a baby kitten has escaped from the bedroom. Joey saw it, his eyes got big, and he ran the other way.
Maybe not. Because the first time he saw kittens was when he accidentally ventured into a room where we were fostering a mom cat and four kittens. He was twice the mom cat’s size, and she was sick. Made no difference. She took off after him like a rocket, snarling. He never came back to the room while they were there.
So, when he sees a kitten, it’s not the kitten he’s scared of. He knows there’s a mom cat around someplace close.