This condition is known as a windowbox effect.
This can occur on a high-definition (16:9 aspect ratio) TV if the video you are watching is a letter boxed picture that was originally designed to be shown on a standard (4:3 aspect ration) TV. Depending on the TV station, this may be seen on certain programs and channels, and is seen often on commercials. It also can occur when watching a DVD that is labeled as being anamorphic.
A 16:9 TV will normally have an option that will allow you to fill the entire screen. Check the remote control that was supplied with the TV for a ZOOM or WIDE button. These buttons can be used to increase the size of the picture. If necessary, refer to the TV instruction manual for further information about the available screen modes. Manuals are posted on your model support page.
NOTE: Some cable and satellite set-top boxes also can control the size of the picture by pressing the Pound (#), ENTER, or FORMAT buttons on the set-top box remote control. If you want to use your set-top box to control the picture, consult the manufacturer of the set-box for more information.