That’s the login screen, not the lock screen. It’s actually GNOME Shell you’re seeing here and following the same rules as GNOME Shell with the same default scaling factor. The only difference here is that it’s running as a temporary GDM user so you always get the default and not a scaling factor saved from a previous session.
In releases before 25.10 you could edit and save the monitor config (~gdm/.config/monitors.xml) of the gdm user to configure the login screen. For 25.10 it’s switched to using a more isolated temporary user for the login screen. So now I’m not sure what GNOME’s plan is to allow you to customise the monitor config of the login screen. Release note added.
This isn’t entirely new though. The main new feature you’re noticing is just that fractional scaling is enabled by default and GNOME choosing a scaling factor to best match its preferred DPI. Exactly the same thing happens when you log in for the first time.