You have 16x9 so black bars are common. You left out the make and model numbers so members can only give generic answers and not be sure if they apply.
Read https://www.google.com/search?q=stream+720p+video+and+I+am+getting+black+bars&ie=utf-8&oe=utf-8 too.
I am trying to stream 720p video and I am getting black bars on the sides of my videos how do I get full screen. My picture is set to 16:9. Thank You for your help.