The neck on a human rig generated using the default human metarig doesn't scale correctly when scaling the root bone.
This happens because the bone MCH-spine.005 has the "inherit scale" box unchecked.
I have provided a .blend file where you can see the bug in action.
In layer 01 you can see the bug (with the bone MCH-spine.005 exactly as rigify creates it, WITHOUT inherit scale), in layer 02 you can see the fix (with MCH-spine.005 WITH inherit scale).
In layer 10 you can look at the metarig used to create the rig (a simple default human meta-rig).