Fully grown chicken hitbox smaller than 1/2 block

I'm trying (in creative mode right now) to experiment with some automatic chicken farming ideas I saw on youtube. The basic idea is as follows:

• Have chickens lay eggs into a hopper, connected to a dispenser.

• The dispenser, with a half slab in front of it, shoots eggs into a chamber half a block high.

• Above the half slab is a water source block.

• When a chick grows into a chicken, its head is in the water, and it suffocates.

• Under the half slab is another dispenser and a chest to collect the drops.

I've tried the lava version of this, and it works great, producing lots of feathers and cooked chicken automatically. But if you want to trade with villagers, you need raw chicken.

Unfortunately, the water version just doesn't seem to work anymore, although I'm not sure what version broke it, since I started playing the game with version 1.10.2. The eggs shoot into the chamber, the chicks are less than 1/2 block high, so they're okay, and they they grow up, and I can clearly see their heads in the water source block. But they're fine. Nothing is happening, unlike what I see in all these tutorials (even those that claim to work with 1.10). So I'm guessing 1.10.1 or 1.10.2 changed something that breaks all these things.