On the primary display, right clicking on screen or pressing 'Z' will toogle between last two screen sizes, for example FIT and 100%. On secondary display, you get the same action by right clicking on screen, but I cannot find any keyboard shortcut.
My idea (request) is to provide a toogle kb shortcut similar to the 'Z' for the primary display. An alternative would be to have 'Carl+alt+shift+0' (100%) toggle between 100% and what 'fit/fill' is set at.