That expression comes from the days when people slept on beds filled with dried grass or hay.

VOA: special.2009.09.20

He opened it by telling the children: "The prisons in our country are filled with people who can't read or write or speak well.

VOA: special.2009.04.27

The inner pot can be filled with fruit,vegetables or drinks.

VOA: special.2009.02.09