ISSUE
If the setting "Use the Big Picture overlay if a controller is connected" is on and you launch a game from Steam desktop in PC, the Big Picture UI scale of the overlay doesn't work. Launching a game with this option and using the quick menu to change the UI scale while playing does nothing, neither does navigating to the Big Picture display settings while playing. The quick menu doesn't even display the adjust bar if you had automatic scale on and then turn it off. Or, if you already had automatic scale off, it will not hide the adjust bar if you now turn it on.

If I open Big Picture first and then launch a game, everything works, however, the changes I make in the "real" Big Picture mode do not affect the UI scale of the in-game Big Picture overlay if I launch a game from desktop.
It's like the Big Picture overlay when launching from desktop is a different thing that cannot use the settings of the "real" Big Picture mode.

OS
Windows 10

STEPS
-Open Big Picture and set a notoriously different UI scale for testing.
-CExit Big Picture and in the desktop launcher, activate "Use Big Picture overlay if a controller is connected"
-From the desktop launcher, open a game.

RESULTS
-The UI scale will probably not match what you set up in Big Picture and even if it does if you try to adjust while playing, it will not work.

EXPECTED
That the Big Picture overlay UI scale settings work while launched from desktop if I'm using "Use the Big Picture overlay if a controller is connected".