Modern TV's usually have rather low native resolutions. As TV is at best broadcast at 1080i this translates to TVs requiring a max resolution of 1920 x 1080 to perform their main duties.
If you have a HD TV, that is not capable of handling full (1080i) HD, then its native resolution may be based on 720p (1280 x 720 pixels).
If you head to System Preferences > Displays, you should see a separate menu appear on the second display, allowing you to set the resolution to whatever you desire. Your TV's native resolution will be the best to look at, but it will always pale in comparison to your Apple LED LCD.