Most modern shows are produced with the 16x9 screen format in mind. Old shows were produced with the 4x3 screen format available at the time in mind. So new shows are going to fill your modern 16x9 screen and the older shows will leave black bars on the sides as they are 4x3 in format. If you watch a Cinemascope movie it will leave black bars on the top and bottom because that format is wider than the 16x9 screen. As the times change, formats change also.