Of course, the most obvious one is the TARDIS from “Doctor Who.” But there’s also Mary Poppins’ Carpet Bag.
There is also Snoopy’s doghouse which was supposed to be a veritable palace as soon as you got past the humble exterior.
I just wondered how many types of things in movies, books, TV, etc. could the CS come up with that are (or are described to be) bigger and more spacious than their obviously humble exterior would allow.
Any suggestions?
The Brady house. 
I remember from playing D&D thirty(!) years ago that a portable hole was awfully convenient. Put the hole down on any surface, fill it with your stuff, then pick it up and be on your way.
There were quite a few places in Harry Potter that “didn’t fit”, mostly Wizardy places concealed in Muggle cities, though the most notable is probably 12 Grimmauld Place.
Harpo Marx’s pockets.
The cinema in *Magical Mystery Tour *, which is inside a tiny pup tent.
House of Leaves, a novel by Mark Z. Danielewski, features a house that grows bigger on the inside than its exterior dimensions allow.
John Malkovich in Being John Malkovich.
Tommy Westphall’s snow globe.
Professor Kirke’s wardrobe.
Oscar’s trash can.
Felix the Cat’s bag of tricks.
I like this one a lot.
Transformers like Jetfire and Astrotrain. They would be about the same size as the others in robot mode, but when transformed into their jet forms, the others could ride inside of them.
The tree occupied by Clothahump (the turtle wizard, not the poster) in Alan Dean Foster’s Spellsinger novels is much larger inside than out, thanks to a hyperdimensional expansion spell. The wizard’s plastron is similarly enhanced, having drawers full of spell components and other things set into it.
But don’t put it inside any of your other extradimensional space devices, like your Bag of Holding, Heward’s Handy Haversack, or Belt of Many Pockets (the only extradimensional storage provider endorsed by Rob Liefeld).
I think 12 Grimmauld Place was just concealed, not hidden inside a warped space. The magic hiding it caused the minds of viewers to “stretch” their perceptions of buildings around it to cover its location. Sort of like the way we don’t perceive the blind spot caused by our optic nerves.
A Bag of Holding is another D&D example.
A couple of other Harry Potter ones spring to mind. The Weasleys’ tent at the Quidditch World Cup, and Hermoine’s handbag in “… Deathly Hallows.”
The Galaxy on Orion’s Belt, from Men in Black.
The holodecks from ST:TNG,* DS9, *and Voyager.
Jeannie’s bottle from * I Dream of Jeannie*
The huge first room in Willy Wonka and the Chocolate Factory.
The house in Heinlein’s “And He Built a Crooked House”, which was a folded tesseract.