The displayed image ratio shrinks because the projector adjusts the displayed image ratio based on a received resolution which does not match the output resolutions the projector is designed for.
Please follow the steps below if you would like to have the displayed image remain intact:
Go to Windows Settings and select Display
Select Advanced Display Settings
Select Display 2: BenQ PJ, then click Display Adapter Properties for Display 2
Select List All Modes, then choose an output resolution that your projector is designed for (as listed)