After upgrading my workstation with NVidia NVS 510 working on four HP LP2475w displays “rotated left” at 4800×1920, I’ve noticed really large fonts in KDE. Using System settings and changing the font size and dpi did not really help, because some apps had after that too small fonts, like KeePass (running via mono). The system-wide solution was very simple, all it took was adding "UseEdidDpi" "false" in the Monitor section of /etc/X11/xorg.conf:

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Hewlett Packard"
    ModelName      "HP LP2475w"
    HorizSync       30.0 - 94.0
    VertRefresh     48.0 - 85.0
    Option         "DPMS"
    Option         "UseEdidDpi"    "false"
EndSection