Lucky Mallet - Shrink Fan Comics

[CinderBH]

Background notes no one cares about:

The mallet is based on the golden mallet belonging to Daikokuten, god of commerce and wealth and one of the Seven Gods of Fortune. With one shake it can make gold coins appear or grant any wish.

It appears several times in popular tales, but most famously in Issun-boushi: at the end of the story the titular one-inch hero obtains the mallet after defeating two oni, and makes a wish on it to become a boy of average height. From that comes the mallet's association with size-changing.