Here's a hint: Mario's in it.
Not much of a hint, because Mario games are generally huge. New Super Mario Bros. is just huger. As of December 26, the game has sold 6.01 million copies in Japan.
This is the first DS game in Japan to break the 6 million mark!
New Super Mario Bros. went on sale on the Nintendo DS back on May 25, 2006. And the game continues to sell and sell and sell. Why wouldn't it?
『New スーパーマリオブラザーズ』の国内推定累計販売本数が600万本を突破 [ファミ通.com]