Wii Fit has sold 4.5M already. It could sell 10M by the end of the year (I doubt it). Nintendo has never (as far as I know) published 2 games in the same franchise within the same 52 weeks.
You can say certainly that Wii Fit 2 isn't comming out in the next 52 weeks from now, as they would consider America/Europe the major launch.
Wii Fit has to average 75K from now on and out, + a big holiday season, and hooray, it's 5M ahead of Brawl.
Also, why did you say "let alone Mario Kart"? MKWii could outsell any other single platform game on Ps360...
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS