I use a 3840 x 2160 screen with GNOME. Due to https://bugzilla.redhat.com/show_bug.cgi?id=1835864 I have been messing around with my appearance settings in GNOME Tweaks a lot the past few days. Somewhere along the line, my Qt settings got messed up and Qt applications started using a font I didn’t like. So I opened KDE System Settings (systemsettings5) and reset fonts to default, then my fonts really got messed up. All fonts became unreadably tiny. For Gtk applications, I got them usable again by setting the font scale factor to 3.0 in GNOME Tweaks, but then Qt application fonts became huge! I tried creating a new test user account and fonts were okay without adjusting any settings. So I tried removing my whole ~/.config directory and it didn’t fix the problem. I have no idea where these settings are stored and what got messed up.
Well, my font sizes are working now after restarting my computer.