From the Super Mario Wiki
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The Minimizer is a magical staff that Bowser only uses during the events of Mario Party DS. In the beginning of the Story mode, Bowser shows Mario and the crew the Minimizer and shrinks them down, similar to the Mini Mushroom's effect. This allows Bowser to take Mario's Sky Crystal. After clearing Bowser's Pinball Machine in Story mode, Donkey Kong and Diddy Kong arrive at Bowser's Castle for dinner. Donkey Kong breaks the Minimizer, reverting everyone back to their normal size. Bowser then fights Mario and the crew without his Minimizer.