ok.. thanks im glad its not just my game/tv/xbox screwing up...i'm getting used to it already

so i just got halo 3 today and i tried to play it on split screen with my brother, but when we played it the screen was smaller than the size of my tv. i dont know why but there are black bars on the left and right side of the screen...i have a 51 inch plasma screen hdtv that runs at 1080i....please help me!