Baby Zombie Glitch

In Minecraft 1.6.2, I built a zombie xp farm from a spawner and saw that 2 baby zombies spawned but when they fell down my collection hole, I could see its face (1 block tall) but it wouldn't run to me (to kill me). Is this a glitch? Or is this implemented? (To make it simpler... it looks 1 block tall but its really 2 blocks tall)