wizard space

Wizard Space is a magical effect.

It causes objects to hold more than their outer dimensions would seem to allow. Wizard Space is created by the Undetectable Extension Charm incantation “Capacious Extremis,”  and is highly regulated by the Ministry (Pm).

References from the canon

•  While not mentioned by name, this magical effect is seen in a number of places in the wizarding world. It would seem that "wizard space" is fairly common, since Molly Weasley didn't seem a bit surprised when their Ford Anglia could hold a lot more people and cargo than it should (CS5).  Since the Ford Anglia fled to live in the Forbidden Forest, no charges were filed against Arthur for breaking Ministry regulations on use of space (Pm).
• (On the other hand, Molly Weasley knew that the car "borrowed" by Mundungus for a Christmas Day visit to Arthur Weasley in hospital had been "enlarged with a spell", rather than having been built like that (OP23).)
• Cauldrons apparently can hold a lot of stuff. Harry dumped an entire collection of Lockhart's books into Ginny's cauldron, for example. And apparently it didn't get too heavy for an eleven-year-old to carry as a result (CS4).
• Harry's invisibility cloak also seems to have this quality, since it can expand to cover several children and a crated dragon, but still can be easily used by a single person (PS14, etc). However, it has limits. When the D.A. was first formed, Hermione pointed out that the invisibility cloak couldn't cover all the members at the same time (OP17).
• The magical tents Mr. Weasley borrowed for use at the Quidditch World Cup were considerably more spacious inside than they looked from the outside (GF7). Perkins didn't want them back because his lumbago was too bad to let him camp comfortably, so Mr. Weasley kept them until he loaned one to Hermione, which proved to be very useful (DH14). That tent was later lost during a raid by Snatchers (DH26).
• Hermione's beaded handbag could hold many books, clothing, and the large Headmaster portrait of Phineas Nigellus thanks to an Extension Charm (which see).(DH9)
• In the early 1900s, Wizards secretly participated in the building of the famous Woolworth Building in New York City, adding ample wizard space for the headquarters of MACUSA (Pm).
• Newt Scamander's brown suitcase held a complete menagerie of magical beasts with various habitats for their survival, including mountains for Mooncalves and a southwestern desert for the Thunderbird (WFT).