Just consider (for a moment) that games like Brain Training, Nintendogs and Animal Crossing all had very poor initial sales on the Nintendo DS mainly because the 'new' gamers they sold so well to are unlikely to buy a game in its first week/month. In general, people expect most of Wii Fit's sales to come from these same 'new' gamers who are still unlikely to buy a game in its first week/month. Selling 200,000+ units in its first week is an amazing sign to what kind of sales we could expect from this game in Japan ...
Quoting myself because I was right!
Nearly 2 Million sales in Japan and it is still selling over 50,000 units every week.