Her laughter, her gestures, her assertions became more violently affected moment by moment and as she expanded the room grew smaller around her until she seemed to be revolving on a noisy, creaking pivot through the smoky air.
Myrtle Wilson had put on a new dress (“changed her costume”) just before this point, and with the dress she put on a new personality: clothes make the woman . . . make her pretentious and vain. Though this is all just a little afternoon party, the sentence is fraught with danger, with “violent” affectation and the claustrophobic feel of the shrinking room, the oversize load on the fragile point of contact. Interesting choice to say the room “grew smaller” around her, but insert the word “shrank” and the sentence doesn’t seem to read right. Something about the pace of the sentence would be suddenly thrown off, it wouldn’t run the silky way that it does now.
On this page, I also like what Mrs. Wilson says when she gets complimented on her dress: “It’s just a crazy old thing . . . I just slip it on sometimes when I don’t care what I look like.“