As of 2020, Zoom used QT’s font scaling to try to adapt to HiDPI displays in Linux based on the EDID reported physical display size. I found this to mean that Zoom’s UI and fonts were way too big, more than double what I’d prefer for my screen.

To fix this, you need to set the QT_SCALE_FACTOR environment variable for the zoom application. While you could do this globally, that will likely mess up other QT applications like VLC. Instead, I recommend setting it for the application only, as e.g.

QT_SCALE_FACTOR=0.5 /usr/bin/zoom

I believe this should work even if you installed zoom using flatpak

QT_SCALE_FACTOR=0.5 flatpak run us.zoom.Zoom

You can also make a custom executable that has this line in it and connect icons, etc, to that new executable; for example,

printf '#!/bin/bash\n\nQT_SCALE_FACTOR=0.5 flatpak run us.zoom.Zoom\n' > ~/bin/szoom
chmod a+x ~/bin/szoom

./szoom