[Ken Maltby]

There is a setting to "Stretch covers and images to fit screen" in the "Screensaver" > "Settings" menu, on the File Browser menu under the Gear Icon.

Calibre also has a Page Layout menu that lets you set its handling of the covers & images to match your device.

Luck;
Ken

EDIT: The "Screensaver" is where most people encounter the full size "cover" image, that is with the default "Use last book's cover as screensaver" setting. If you were talking about the results of a long press on an image; it's the default to scale the image to "fullscreen" (there is a row of buttons on the bottom for some options as well as the image)