Unused room in the suitcases of the rich comes at the expense of items of little consequence. The suitcases of the poor get full while they are still packing items they very much need. Space is at a premium in the small suitcase, whereas its limits matter less when suitcases are big. Economists call this diminishing marginal utility: the more you have, the less each additional item is worth to you. There is an almost economic logic to all this: the poor have less slack because they can afford less of it.
