Why is my Ragdoll so heavy headed?

I've just followed a couple of tutorials to get my enemy model rigged with a ragdoll. When I kill the enemy I deactivate the animator component causing the ragdoll to take control of the physics and causing a collapse to the floor. Whilst this does work, it looks as though the Ragdoll is "too heavy" and slams to the floor like its turned to lead. I have tried reducing the mass in each of the rigidbodies but without much luck - tends to make him faceplant the floor regardless of the head mass being 0. Any ideas where I've gone wrong? Cheers