HDMI is a pixel-per-pixel display. This means that whatever information is being sent, it will display. The most likely culprit is the video card, in which case, getting the latest driver will likely do the trick.
16:9 is the aspect ratio, and to fill the screen, you'll need a 1920x1080 source content. If the resolution of the content is less than that, it will display like you describe also.
You might also want to change the name of the HDMI input to "PC".
I have a Samsung LN46A650 LCD which is hooked up to a Media Center PC using HDMI. The problem i have is that the picture displayed on the TV does not fill the entire display. It leaves about 2 inches black on the left and right side, and about 1 inch black on the top and bottom.
I have it set to 16x9 and Windows7 Is showing this as the native best resolution to use for this display. I cant figure out how to make this stretch to the full display, its literally wasting several inches. I have tried everything i can, gone through the menu several times, i have tried to do a firmware update, and all without much luck.
What can i do about this, i really want to fill the display with the native resolution.