Screen Share of Monitor has been shrunk so this increases the black border around the outside

[RMRL]

Can someone advise why my screen recording sometimes reduces the size of the screen share, leaving a bigger black border around the outside. I have adjusted the settings so that people's videos don't overlap my screen so I understand that there will be borders but on some recordings it's a though the recording zooms out hence leaving a massive black border around the outside.

The first image shows how it normally looks.

The second image shows when the screen share has been shrunk.

In case it helps, my set up is that I run Zoom on my laptop and I screen share pdfs, word docs, etc from an external monitor.