Hello.

After having tried (with no success) to properly configure a multi-screen, multi-definition setup, notably by installing plasma-wayland-session and plasma-wayland-protocols and fiddling with the Plasma settings in the Wayland session, some GTK application fonts are now very small when reverting back to the default Xorg.

The "System Settings -> Appearance -> Fonts" and "System Settings -> Application Style -> Gnome / GTK Application Style" panels don't have any effect on this behavior. Note that I don't see any option regarding GTK fonts in the later panel.
Some system info:

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❯ inxi -z -Gxxx -S
System:    Kernel: 5.13.1-3-MANJARO x86_64 bits: 64 compiler: gcc v: 11.1.0 Desktop: KDE Plasma 5.22.3 tk: Qt 5.15.2
           wm: kwin_x11 vt: 1 dm: SDDM Distro: Manjaro Linux base: Arch Linux
Graphics:  Device-1: Intel CometLake-H GT2 [UHD Graphics] vendor: Gigabyte driver: i915 v: kernel bus-ID: 00:02.0
           chip-ID: 8086:9bc4 class-ID: 0300
           Device-2: NVIDIA GA106M [GeForce RTX 3060 Mobile / Max-Q] vendor: Gigabyte driver: nvidia v: 470.42.01
           bus-ID: 01:00.0 chip-ID: 10de:2520 class-ID: 0300
           Device-3: IMC Networks USB2.0 HD UVC WebCam type: USB driver: uvcvideo bus-ID: 1-9:3 chip-ID: 13d3:56a2
           class-ID: 0e02 serial: <filter>
           Display: x11 server: X.Org 1.20.11 compositor: kwin_x11 driver: loaded: modesetting,nvidia
           alternate: fbdev,intel,nouveau,nv,vesa resolution: 3840x2160~60Hz s-dpi: 192
           OpenGL: renderer: Mesa Intel UHD Graphics (CML GT2) v: 4.6 Mesa 21.1.4 direct render: Yes


Note that my UHD 15.6 inches display is definitively a source of problems for Linux DEs in general, but I had no problem with fonts before.
Some relevant files I could find:

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❯ cat ~/.config/gtk-3.0/settings.ini
[Settings]
gtk-application-prefer-dark-theme=true
gtk-button-images=true
gtk-cursor-theme-name=Vimix-white-cursors
gtk-cursor-theme-size=48
gtk-decoration-layout=icon:minimize,maximize,close
gtk-enable-animations=true
gtk-fallback-icon-theme=Adwaita
gtk-font-name=Noto Sans, 10
gtk-icon-theme-name=Papirus-Dark
gtk-menu-images=true
gtk-modules=colorreload-gtk-module:window-decorations-gtk-module
gtk-primary-button-warps-slider=false
gtk-theme-name=Aritim-Dark-GTK
gtk-toolbar-style=3


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❯ cat .gtkrc-2.0
gtk-enable-animations=1
gtk-primary-button-warps-slider=0
gtk-toolbar-style=3
gtk-menu-images=1
gtk-button-images=1
gtk-cursor-theme-size=48
gtk-cursor-theme-name="Vimix-white-cursors"
gtk-icon-theme-name="Papirus-Dark"
gtk-font-name="Noto Sans, 10"


10pt looks normal to me, and is the default size of my KDE font:


How things look:
Firefox top bar:

Thunderbird:


For comparison:




I have reconnected/rebooted a few times after having deleted some files/changed settings (including global scale, fonts, GTK theme). I don't really now where to look now...

Well, not sure how I did this, but changing the global scale factor and rebooting again a few times did the trick. I think that it was still a bug though, given how the Qt apps had the correct ratio and not the GTK ones...