I keep thinking of how he could have dressed Scout differently to make her look more sophisticated-adult-ish. I think one major problem was with his fabric choice for the bodice; that eyelet was downright juvenile. (The whole thing skewed sweet to me, when I would have really responded to badass, as more in keeping with who she seems to be, and more aesthetically in line with that gorgeous prosthesis.)
Another problem was that he put her in a bodice that was straight up and down. He followed her natural figure -- and a fine figure it is -- rather than choosing seaming that might have suggested curves, one of the things that cue us that we're looking at a grown-ass adult.
I also know that women with a smaller bust sometimes choose styles that give an illuuuuusion of fullness up top, like a ruffle or pleats or a collar treatment with a bit of volume. Absolutely nothing wrong with flaunting one's flatness, but if Marquise had considered it, he could have asked her what she thought about adulting things up with that kind of embellishment.
In general, I loved seeing these women celebrated, and it was intriguing to see the designers cope with bodies that weren't like their usual models'; each of the women had, I think, pronounced physical differences that the designers had to negotiate.