The automatic zoom feature is not functional correctly. It can only shrink a large image to fit the screen, but failed to enlarge a smaller image to fit the screen. "Keep Zoomed" is not a good choice since the image may differ in size.
And there is a potential bug in full screen mode with auto-zoom enabled: press Opt+0 will enlarge the image to fit the screen, but it will automatically shrink back to actual size the moment you release Option key. Sound like a joke, but it did happened.