You can add the "-forcedesktopscaling" variable to your startup, specifying a numeric value.

For example, for Fedora/KDE Plasma I have the 'Command-Line Arguments' field, for the Steam entries, in the 'KDE Menu Editor' app, to say "%U -forcedesktopscaling 1.25".

The old GDK_SCALE variable is no longer being looked at by the Steam client (as far as I can tell, and in case you were using that variable).

Same issue. I think it has something to do with the recent update enabling GPU acceleration. I'm using KDE with 225% scaling, but Steam doesn't respect it as of now.

Oddly enough I no longer have that issue since the update (although it seems to come and go, and be present in the beta but not the ““stable”” client or vice versa). Plasma/Kwin Wayland scaled to 200% on UHD.