Symbolism, Imagery, Allegory
In Alice's Adventures in Wonderland, Alice changes size constantly. When she first arrives in Wonderland, she's too large to make it through the little door into the beautiful garden; after she drinks from the mysterious bottle, she's too small to reach the key. Once she eats the special cake, she's enormous, but the White Rabbit's fan makes her small again. In the rabbit's house, another bottle of mystery cordial makes her swell up and get stuck in the room; pebbles thrown in the window turn to cakes and eating them shrinks her. At this point, Alice meets the Caterpillar, who teaches her to use pieces of mushroom to control her size – nibbling a bit from one side to get larger and a bit from the other side to get smaller. Just when we think she's finally in control of her size, she begins growing in the courtroom, getting larger and larger until she eventually realizes that the characters around her are just a pack of cards.
Why all these changes in size? What does it all mean?
Well, we have to make the obvious connection between size, age, and maturity. Even back in the real world, Alice is constantly changing in size because she's a growing girl, getting a little bigger and a little older all the time. This is an arbitrary process that she can't control; all children grow up. (Unless you're reading a different book called Peter Pan.) It's also a process that makes Lewis Carroll, who was fond of children but not of adults, nervous. Alice changes size – and changes in relation to everyone around her – because Carroll wants to show us that growing up is unpredictable, sometimes just making you feel awkward, sometimes actually putting you or others around you in danger.
In Through the Looking-Glass, Alice doesn't really change in size, although the chess pieces and game themselves grow large. However, Carroll draws our attention to her age and process of maturing. When Humpty Dumpty tells Alice, who is seven and a half, that she should have "left off at seven," he seems to be speaking Carroll's own thoughts. Couldn't she just stay a cute little girl, ready to listen to fairy tales, forever? Of course not – the only way to freeze your age in time is by dying!