Extension Charm

The Extension Charm, cast as Capacious Extremis, makes the inside of an object larger without changing the outside, so it holds far more than it should and carries the load more lightly. It is advanced magic, and the Ministry of Magic keeps it under tight control, since a bag that swallows a wardrobe is exactly the sort of thing that breaks the International Statute of Wizarding Secrecy in front of an audience. The law allows it only for manufacture, on objects individually approved by the relevant Ministry department, and not for private casting. Most wizarding luggage carries it, Hogwarts school trunks included, along with family tents, which is why a tent pitched for two can hold several rooms, and the official cars the Ministry runs. In theory a hundred wizards could take up residence in a toilet cubicle, given enough skill with the charm.