It’s possible your TV remembers different settings for different inputs but that’s assuming that it’s not all going through the same one.
Based on what you said I’m guessing it’s a setting in your BD player. My Oppo has various stretch/zoom options. There is probably a menu selection that tells it to retain the original format and use black or grey bars on the sides.
I've looked through the owners manual of the AVR and BD player and couldn't find anything....
An internet search is often more productive than trying to figure out a Denon manual.
