This is because of how Steam recognizes high resolution monitors and scaling in Xorg. You will have to edit your Steam shortcut so that it uses it's own scaling as it doesn't honor the system scaling.
To do that:
sudo nano /usr/share/applications/steam. Desktop
change Exec=/usr/bin/steam %U to Exec=env GDK_SCALE=2 /usr/bin/steam %U and save it.