I think the head is fine at it's current size. Actually, I believe it could be a little bigger and here's why: IMO the current head represents Leech in attack mode when he is bleeding a victim/foe. In this case similar animals like snakes, pythons and other animals dislocate their jaws making their heads bigger to ingest their prey. I always viewed Leech in the same vein. What would have helped would have been a regular "at rest" head for Leech that would have been more proportionate to the rest of his body.