I'm having the same issue with Monkey Ball:Banana Blitz. I have only played that game, wii sports, and wario ware on my tv and both of the games published by Nintendo fill up the entire screen, while the 3rd party game has the bars. I noticed this problem before I bought the component cables, so I don't think it has anything to do with the cables or the wii itself. FYI, I'm using the React component cables (only one I could find) and my tv is a Samsung HPS4253. I'm also running the wii through my Yamaha receiver, but I've played other sources through the receiver without this problem so I don't think that's it.
I had previously noticed a similar problem while playing my gamecube on the 16:9 tv. Of course, I never had component cables for the gamecube but the black bar problem occured on every gcn game (1st & 3rd party).