For humanoids that have a head with MeshType of Sphere, the head is not scaled properly when setting the HeadScale parameter. (I have not checked to see if this bug occurs for other MeshTypes - so you may want to check other MeshTypes as well).
For example, in the screenshots below, the HeadScale is set to 3, but the actual head (head on the left) is much larger than the expected 3x size (head on the right).
