For KDE font/gui size issues after switching to NVIDIA driver, I found the following post:

It’s a nvidia thing lol I had the same issue and never even noticed it until one day I gave the free noveau drivers a try and everything looked way better and smaller. For some reason nvidia sets/forces my DPI to I believe 120, anyways it’s really easy to solve.

Open the nvidia.conf file in your text editor

sudoedit /etc/X11/mhwd.d/nvidia.conf

Then edit or add the option DPI under the screen display sub section and disable UseEdidDpi.

To make things easier here is what I have under my “Screen” section which now forces the use of 96 for dpi scaling.

Section "Screen"
    Identifier "Screen0"
    Device "Device0"
    Monitor "Monitor0"
    DefaultDepth 24
    SubSection "Display"
        Depth 24
        Option "UseEdidDpi" "False"
        Option "DPI" "96x96"
    EndSubSection
EndSection

I hope this helps you as it did for me.