Why did his wife specifically choose the example of his belt?

Maharsha: Their custom was to keep money in their belts. She called him 'Empty One' as to be able to pay back all of the victims of his theft, even by emptying out all of the money in his belt together with the belt itself, it would not be enough.

Ein Eliyahu: The Gemara in Moed Katan 14a speaks of a person who is so poor, that he only has one tunic. He may wash it on Chol ha'Moed. The Gemara questions how onlookers would know that he has special permission. The Gemara answers that his 'belt' proves it. While he is washing his tunic, he wears his outer cloak and puts his belt on top of it. His wife was saying that even if you gave back everything and reached the poverty of someone with only one tunic, who uses his belt while washing it, you still would not have returned everything.

Mishnas Eliezer: The Yerushalmi (Yoma 7:3) writes that the Avnet (belt) of the Kohen's vestments would atone for thieves. It is worn opposite the heart and atones for thieves who 'steal the hearts' - i.e. fool, their victims. The man's wife was indicating that even the atonement of the belt of the Kohen cannot help you because your theft is so great.