SUMMARY
Font sizes are inconsistent across KDE programs.

STEPS TO REPRODUCE
1. Upgrade plasma-workspace from version 5.17.4-1 to 5.17.5-1
2. Relog
3. Open Dolphin, Elisa, Kate or Konsole

OBSERVED RESULT
Whenever you open these apps, the font is larger that everywhere else, which makes the interface look distorted. It doesn't happen anywhere else: System Settings, Info Center, and all non-KDE apps that I've tested, such as Chromium and Audacity, are completely unaffected.

I've tried everything, from changing fonts, font sizes, anti-aliasing, forced font DPI, scaling... When I mess with these settings, I can eventually get the affected apps to look normal, but then everything else will look too small.

I was finally able to pinpoint the issue, after a lot of downgrading packages, and the culprit is the plasma-workspace package. Versions 5.17.4-1 and below are fine. 

EXPECTED RESULT
Font sizes should be consistent across programs.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kernel 5.4.10-arch1-1, Arch Linux
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.65.0
Qt Version: 5.14.0

ADDITIONAL INFORMATION
Resolution: 1920x1080
Global scale: 1.5x
Font type, size: default
Font anti-aliasing: enabled
Force font DPI: 144