Photos suddenly not appearing full screen in camera roll

Hi-- I was viewing my photos in the camera roll and things were working fine. Then all of a sudden (I don't know if I pressed something or what) the pictures are now appearing smaller with black space around most or all of the edges, no matter what view I have it in or how I'm holding the phone. Album pics show up the same way now. I can't get them full screen unless I manually zoom but then when I scroll to the next picture it's cropped again. I couldn't find help for this anywhere. How can I get it back to how it usually is, where the pictures fill the screen automatically? Any help is greatly appreciated.