The SDDM login manager and its various widgets and fonts looked too small on the UHD screen while in native resolution.

So I uppedd the DPI setting by:

# https://wiki.archlinux.org/index.php/SDDM

[dhaskew@xps13 ~]$ cat /etc/sddm.conf | grep ServerArguments
ServerArguments=-nolisten tcp -dpi 184

Adding the “dpi” option made a world of difference.