Little Luffy found a box in the closet.

In it was a big pair of shoes.

This confused Luffy. He was certain that these things had never been in the hall closet.

Once he stuck his feet inside, they felt humid as if they had recently been worn. Comparing the length to his own shoes revealed to him that the owner had feet about four times his size.

Years later, when his feet fit, he considered taking the not-so big pair of shoes with him on his adventure.

However, Monkey D. Luffy just wanted to fill his own large shoes.