Problem with weights/posing

So, lets say, I have a simple model - neck, head, and eyes. Two bones for eyes, parented to head, head parented to neck. Everything is weightpainted correctly - eyes influence eyes only, head influences head only (so the neck part is blue), neck influences neck only (so the head part is blue). However; when i rotate the head in pose mode, only eyes rotate along with 1.0 weight, the head mesh rotates slower (so the eyeballs pop out ahead, lovely) and shrinks down to 25% its size when i rotate by 180°. Why is that???? Thanks.