First off, does your TV have a "Crop", "Stretch" or "Wide" setting you happened to turn on? If your using your Wii on the TV with that feature on, depending on the TV it may crop or stretch it (verticaly or horizontalty) too much (or too little in most cases). Though most TVs i've seen have a just a zoom feature that does nothing to the black boarders other than a do a close-up on the image.
Second, what sceeen settings are currently set on the Wii? if you have 16:9 on without the crop or stretch feature actavated on your TV, it may or may not fill the screen all the way, with it on, it may do what I describeded ontop.
Do you have also have that issue with yer X-Box 360 while playing on your TV the feature on? Most Xboxes i've seen have a auto-adjusting aspect ratio that will fill the screen perfectly even with the Tvs stretching or croping features on, so it shouldn't be a problem with your box.
Try what you can and see what works, report back it you got any other questions or if anything is in working order for ya.