Explain the significance of Mathilde's reflection upon losing the necklace: "How small a thing is needed to make or ruin us!"

The statement is full of irony. Losing the necklace, she believes, ruined them. It is her vanity that was what ruined them, not the necklace. The mere perception of the necklace is what she admired and wanted. If she had not wanted the illusion of riches, they would not have lost everything.