Symbolism, Imagery, Allegory
Be honest. Did you feel yourself getting claustrophobic when you read about the teeny tiny spaces that Arrietty's family lives in? They live far under the clock, sheltered from the upstairs world by an intricate series of gates that only Pod knows how to open.
Yikes. No wonder Arrietty feels cooped up with her parents. How would you feel if the only people you ever saw your 'rents, and the only places you ever saw were the rooms in your own house? At best, it's a snooze fest, and at worst, it's a serious case of cabin fever.
Why all the secrecy and protection? And just what are all those gates doing between Arrietty and the outside world?
We, like Arrietty, at first think that the gates are just to protect them from all the harmful things out to get her, but we soon find out that the gates exist for something a bit more sinister:
"Strong gates," said Pod, "gates you can't open. What are they there for?"
"Against the mice?" said Arrietty.
"Cats?" echoed Arrietty, surprised.
"Or to keep you in?" suggested Pod. (6.17-18, 23)
You mean the whole time those gates exist as physical barriers to Arrietty's freedom?
No wonder Arrietty cries, repeating the same words over and over: "Arrietty put her face into her hands. 'Gates…' she gasped, 'gates, gates, gates…'" (6.55).
Gates and locked doors (think of the boy in chapter 18) are physical reminders of the barriers that keep characters from claiming their independence, but they usually just make them want to break free all the more. Right after Arrietty learns that the gates are to keep her in, she begins her rebellion against her parents for freedom, and as soon as the boy gets locked in his room, he performs his greatest act of heroism.
In other words, the more you try to cage them in, the less you can hold these free spirits down.