>>396952724
Yeah that's an issue, you can scale the head mesh instead. Not sure why it doesn't just behave when you scale the head.
Alternatively, I made a python script for 2.79 (might work in 2.80+? idk) that can scale shapekeys for you. So if you made the head 0.85x scale in total (taking into account scaling of the spine/chest/neck/head and stuff), put 0.85 in the "modif" variable at the top and run it in Blender in a text window.
https://pastebin.com/E84pAuRZ