You may be watching an older program that was filmed in a 4 to 3 aspect ration and have a picture format setting that maintains the original aspect ratio of the signal as opposed to stretching it to fill your TV screen.
Availability of format options differs based on your TV model and input video signal resolution and type. Please read the user manual of your TV for specific information about your TV’s format options.
Automatic or Normal picture formats produce the best looking image format without distorting the aspect ratio of the input video signal.
Automatic or Normal picture formats use the dimensions of the video signal (i.e 480i, 720p, 1080i, 1080p) vs. the dimensions of the actual picture within the video signal, which can be in numerous different aspect ratios to display a picture on your TV screen. This prevents the aspect ratio of the image from being distorted (where people look short and pudgy, and circles look like ovals). However, older programs that were filmed in 4:3 aspect ratio or which are letterboxed and/or pillared may sometimes not fill your entire screen in these modes.
For such a signal, if you want an image to fill more of the screen you can use other formats such as Zoom, Super Zoom, Movie Expand, Movie Expand 14:9 or Movie Expand 16:9 based on your personal preferences and their availabilty on your TV model. These other formats are especially useful when the entire program and not just commercials are being letter-boxed and pillar-bared at the same time (which cause black bars on all four sides).