If you ask a fan of Doctor Who to describe the Doctor's time machine, the TARDIS, in one sentence, they would very likely use the phrase that most characters in the show use upon first encountering it: "It's bigger on the inside." Externally inspected, it appears to be an ordinary British policeman's box from the 1960s; but, thanks to the technological feats of the people of the planet Gallifrey, inside it is nearly limitless in size, holding guest rooms and wardrobes and libraries and swimming pools and laboratories and, for an engine, an artificial black hole. It's representative of the show's whole charm: things aren't what they appear, they have a deeper secret to be uncovered--a silly little man with a blue box turns out to be a 1,200-year old Time Lord with the most powerful machine in the universe at his disposal. But to discover that, you have to trust him. You have to step through the door to learn that it's bigger on the inside.
I always thought that phrase sounded familiar. Then I remembered I'd heard it before! In two places, actually. One is in C.S. Lewis' book The Last Battle, from the Chronicles of Narnia series. The book's characters come to a walled garden, but once they enter its gates they find it's an endlessly expansive world in itself, "bigger on the inside." From the outside the boundaries of the garden could be clearly seen; but from the inside, the characters discover they can forever go "further up and further in." Lucy notes that once, in our own world, there was a cave that was bigger on the inside, too--meaning the cave in Bethlehem where Christ was born, where a little manger held a tiny babe who was the infinite God. With both the garden and the cave, you have to enter to discover it's bigger on the inside.
The other place I had encountered this phrase was in G.K. Chesterton's book The Catholic Church and Conversion. Chesterton says that the non-believer or non-Catholic will look at the Church and see an admittedly large and old human organization, but nothing more--no different from the nation of China, for example. But if you enter its doors you step into 2,000 years of tradition and belief, and a spiritual history that stretches back to the Garden of Eden; you step into the heavenly liturgy itself through the bridge of the Holy Mass; you come into the very presence of God in the Blessed Sacrament, itself an example of an apparently small thing holding an infinite reality within it. When you approach the Church and its mysteries with the eyes of faith, you are able to perceive it in all its glory and majesty and wonder. Thus, "when the convert has entered the Church, he finds that the Church is much larger inside than it is outside."
A madman with a blue box. A lion with a gated garden. A babe in a cave. A small wafer of bread. Each contains a secret: they're bigger on the inside. But to see it, you have to trust them.