That's not how breasts develop, though... Usually the first bras you buy are not lingerie but look more like sports bras
Also it's a bit concerning that this clothing store sells erotic lingerie for kids who are around 10 years old!
Maybe this is set in a world like that of Satsuki-san is an Apartment Wife, where the legal age for consent/marriage has been lowered to "when you definitively hit puberty" (translating to first menstruation for girls), and somehow it actually worked out with little to none of the hiccups that would ensue if this was done IRL.
This was clearly a specialty lingerie shop, and some Japanese women are tiny; I don’t think the erotic gear was specifically aimed at children. Yukari just picked those up because she naively thought that because they were so minimal her classmates wouldn’t notice them under her clothes.
Some lingerie for grown women would fit in a cigarette pack.