A baby zombie can't pass in a 1 block hole if there a carpet in front of it. (I guess it'a a half block, but i didnt test it with a slab or something else. So i dont know if it's only with the carpet)
But a Chicken Jockey is able to pass inside this hole!
It's clearly a bug since the chicken jockey must be bigger than the baby zombie alone 😃 (Or the chicken grease him up!)
Not necessarily a bug. Mobs just don't account for the height of their riders, and just mercilessly let them suffocate when walking through a hole. A chicken is smaller than a baby zombie, so it should fit through alone.