The other options did not directly work for me, however Luke Hsiao's tips  did.
With a Flatpack in Pop!_OS 20.04, I changed my
~/.local/share/flatpak/exports/share/applications/us.zoom.Zoom.desktop to include
QT_SCREEN_SCALE_FACTORS env in the exec line.
First I ran
xrandr to find out my main screen is HDMI-A-0.
Then I prepended the env to the Exec line
Exec=env QT_SCREEN_SCALE_FACTORS=HDMI-A-0=2.00 /usr/bin/flatpak run --branch=stable --arch=x86_64 --command=zoom --file-forwarding us.zoom.Zoom @@u %U @@
If you have more monitors you can append their settings as comma-separated values.