If the TV box is HD it may be sending a 16:9 picture with black pillar bars included in the frame for 4:3 content.

An upscaling DVD player will usually do the same. But if the DVD player is only outputting 480 lines it'll be assuming the TV is expecting a 4:3 picture.

The TV's default mode will be to fill the screen no matter the picture's aspect ratio.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris