From the Super Mario Wiki
The Mini Barrel

Mini Barrels[citation needed] are devices in the game Wario Land: Shake It!. They are somewhat rare, only appearing in two levels: Bad Manor and Boogie Mansion. Each Mini Barrel resembles a brown barrel with two yellow arrows and two openings, one big and the other small. When Wario enters the large opening, he comes out of the other opening as Mini Wario. Mini Wario can fit through small passages and find hidden treasures. When Mini Wario enters the smaller opening of the barrel, he turns back to normal. However, the barrel has no effect on Wario if he enters the large opening in his Mini form.