Symbolism, Imagery, Allegory
Puffed sleeves, omg.
It's all Anne can think about when she imagines herself as different from the orphan she is in the beginning, wearing a too-tight wincey dress. If you're wondering what wincey looks like, here's a hint: you can't have wincey without the word "wince." It's a coarse, scratchy fabric, and if that isn't bad enough, we're told that Anne's dress is a cheap wincey, with a yellowish-gray color. So Anne starts out the book wearing the most awkward dress imaginable.
In Anne's imagination, puffed sleeves represent the opposite of what she's used to: a world of luxury. The sleeves are extra material, "big as balloons," (25.32) Marilla says. They're more than needed when, with her tight dress, Anne is used to less.
Too bad for Anne. Between her orphan dress and puffed sleeves comes a middle phase: the three dresses Marilla makes for her after the Cuthberts decide to keep Anne. They're a step in the right direction; they fit her. But Anne's not going to be in a fashion show anytime soon.
To Marilla, fashionable clothes represent sinfulness and vanity. To Anne, and to Matthew once he finally notices, they represent fitting in. Anne doesn't look like the other Avonlea girls until Matthew has a dress made for her with puffed sleeves. Marilla then relaxes her rules, making Anne fashionable dresses to avoid Matthew going behind her back to get them.
She seems to realize fashion isn't so morally damaging, either. When Anne thanks Marilla for putting a flounce on her dress, saying,
"I know I'll be able to study better because of mine. I shall have such a comfortable feeling deep down in my mind about that flounce." (31.9)
Marilla admits,
"It's worth something to have that." (31.10)
We see your point, Anne. Vain or not, it feels good to look good. And the time Anne spent getting all jelly of the other girls is now freed up for more important things.
And let's not forget that Anne dressing like the other Avonlea teens is another way she now fits in. More and more, she's becoming an accepted member of the community.