I actually switched today on my PC and yesterday where i wrote this on my Laptop.
So im 100% sure since i saw directly the smaller icon’s before and after the update, that its reduced from the new KDE 6.5.3 version on my second device.
I actually had here a screenshot from the Taskbar Menue/ Application Launcher, which i used on both devices because from another KDE (X11) bug which i had today with my PC: