How to shrink (?) iMac's screen?
It doesn't seem to be possible to underscan an internal display on macOS, only over HDMI for external displays.

Underscan/Overscan is specifically an HDMI thing, as far as macOS is concerned.
I dug through the workings of the core frameworks responsible for display manipulation like two years ago and there was no way to accomplish what you're after. Like it's just not supported. You'd probably have to write your own graphics drivers to accomplish it, and I don't think that's terribly feasible. Sorry. :/

How do you Underscan/Overscan the internal display on a mac? - /r/osx