Thin film mirrors or solar sails. Just a micron or so thick. So you could package square kilometers of material, so long as you can figure out how to unpack them. The thin film mirrors could be used to focus sunlight onto solar panels in space. The solar sails could be used for navigating through the solar system.
Or, very long tethers could be hundreds of kilometers long. Could be used for instance for artificial gravity - two habitats rotating around each other by a tether system.