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 allowed Bowser to take Mario's Sky Crystal. After clearing Bowser's Pinball Machine in Story mode, Donkey Kong and Diddy Kong came for dinner, but Donkey Kong breaks the Minimizer and everyone was back to normal size, and Bowser fights Mario and the crew without his Minimizer.