When I change my desktop density to something else than the default one, that setting only comes into affect after I logged in. In the login screen (where I need to type in my password) the system-default screen density is used (at least I guess so), although I am the only user of the system.
It's a known issue. We can't use your user settings because we don't know which user you are yet in the login manager. There's a bug report upstream (see github on SDDM) including some workarounds.