Fit to screen when mirroring iPhone screen to Macbook Pro using Quicktime

I'm using a Lightning to USB-C cable and Quicktime's in-built "New Movie Recording" function to mirror my iPhone 11 Pro Max display to my 2016 Macbook Pro. Even when I go into full screen mode, the mirror takes up only about one third of the space that would be available on the display. The rest is taken up by black bars. How do I get the mirror to fit the screen and fill it out as much as possible? Ironically Quicktime does have a "Fit to Screen" option in "View", but it still includes huge black spaces/bars framing the actual mirrored display.