when I rent hd on demand movies they have a black box on the top and bottom
Sometimes in your tv settings you can change the view. Cinema, music, standard, hi contrast, stretch etc. additionally there can be 1080p or 780i etc. which can show the media larger or smaller.
read the manual for your tv first. When I had cable there was settings like I mentioned above in the set top box which could be changed.
Read about OAR (orignal Aspect Ration). Most movies are not 16x9. (or the old SD standard of 4x3). Personally I am happy that rental movies are not automatically chopping off the edges of a picture to zoom it to fill a screen.