What's wrong with my new HDTV if the HD picture doesn't stretch all the way across the screen? ?

I have a new Samsung HDTV 46 inch that works fine, pictures from any channel including HiDef and regular broadcast fit the screen to its edges. I just got a 26 inch HDTV and some tv shows fill the screen and while others do not. The installer said its non-high def shows that show at 4:3, but three are many 100% HD channels that have an 4:3 aspect ration instead of 16:9. Could it be the comcast converter box from Comcast since the working TV has a different converter box?