PBP, or Picture By Picture, provides you with two displays from two different input sources at one time. If you would like to scale both displays to full screen in PBP mode, please setup resolution settings for both as 8:9 (1920 x 2160) in OS display settings, applying to monitor main screen and sub-screen.
In MacOS: go to the Apple menu > System Preferences, click Displays, then in Display Preferences press “Option” and click “Scaled” at the same time to unlock the full resolution list, and then choose 1920 x 2160. If the OS does not show this resolution, it needs 3rd party software to add it.
In Windows: enter NVIDIA/AMD Control Panel or Intel UHD Graphics Control Center to set the resolution to 1920 x 2160 for each screen.
*Please note that whether special settings can be used or not depends on graphics card driver and operating system support.