When you plug in the rMBP to the TV, find the system preferences and make sure that the display setting is on 'Best for External Display'. (I'm assuming you are using mirror mode, but this should work for either or)
Once you have made sure you are on 'Best for External Display', find the remote for your television. Make sure it is the actual TV remote, don't get confused by your cable remote, you need the actual TV remote that came with the set.
On the remote find a button (usually near the bottom, for mine it's actually right on top) that will allow you to change the aspect ratio of the TV. I forget what it is usually called, something with a P.. I think. Maybe PIC, or something along those lines, you'll know it when you hit it because your TV aspect ratio will start changing. Mine's called Pixel I think... anyway depending on the type of TV you have you should be able to look up where the button is online if you really can't find it.
Also depending on your TV, you will have a few options for aspect ratio. The one you are looking for is the most zoomed OUT aspect ratio. On Samsungs, it's usually Just Scan, or perhaps Wide 16:9... On Toshiba it's called Native (NOT Full). You'll notice some of these ratios are very similar to others. Make sure you use the one that allows you to see the entire (native) resolution of the screen.