This reddit thread discusses what sounds like the same issue: https://www.reddit.com/r/kde/comments/1190vom/small_fonts_after_switching_to_527/ Which refers to https://wiki.archlinux.org/title/HiDPI#KDE_Plasma and the fact that Plasma 5.27 dropped use of GDK_SCALE/GDK_DPI_SCALE variables. But I'm seeing small KDE fonts as well as GTK fonts, so that might not be the whole story.