Flying from Sacramento to Newark involves two planes and a transit in the never popular O'Hare airport.
It strikes you immediately that inside a plane or an airport there are countless objects--people, clothes, food, conversations, all milling about differently.
In a flattish ontology where we don't discriminate against medium-sized objects in the name of the tiny (undermining) or the vast (overmining) we have to accept a truth that Levi refers to as strange mereology: there are more parts than wholes.
Or to put it another way, the inside of an object is bigger than its outside.
This startling intuition is just one way in which OOO escapes correlationism, reductionism and holism in one fell swoop. If you like it means that a feature of the Kantian sublime--inner space is bigger than outer space--is extended to all entities.