Well, this is a completely new #Linux issue for me. I updated my @pop_os_official, @COSMIC_desktop, install earlier, then switched the machine off to take a break.
When I switched it back on, however, the mouse cursor has changed. In Brave, Vivaldi and Spotify, it becomes really small... but in LibreWolf and my note apps it's perfectly fine. This will be an interesting Easter challenge to try and solve!