I built a vanilla chicken farm in ATM8. When eggs are dispensed onto a slab, and a baby chicken spawns, the baby is 1/2 block tall. When it grows up, it's expected to be taller so that it'll intersect with lava above and die from fire damage. Unfortunately, in ATM8, the adult chicken height is still 1/2 block tall, which I can verify by turning on bounding boxes (F3+B), where the bounding box is still 1/2 block tall. I can't find any reason why the adult chicken height should be different in ATM8 compared to vanilla, so this seems like a bug.

The bounding box height looks half a block tall, and the yaw indicator is lower than they eye height, but it appears I was mistaken about the actual height being 1/2 block.