If your mplayer doesn’t seem to be zooming into, or expanding a video to fit the screen under fullscreen mode (Shows black bands all around the video, preserving its actual size), then it’s probably because of the video driver your mplayer is using. I had set mine to xv sometime back in order to play a few Real media files, and though it did zoom in on those files it confounded me when I tried other normal video files like AVI, etc on it.
The solution is to switch the video driver (Preferences – Video tab) to x11, which supports zoom.
