The --force-device-scale-factor flag seems to have issues sometimes as reported here. Alternatively manipulating /data/volumiokiosk/Default/Preferences as described in the linked thread has the risk that the structure / content of the Preferences file could change on any update of the chromium browser. While this would not be an issue for the current Jessie based Volumio (because there will most probably no more updates for chromium browser 
) this could change when Volumio will be based on Buster which is in development right now. I will think about your request, but IMHO the better way to tackle the issue would be an option in Volumio itself allowing to scale the UI.