Far be it from me to expect realism in my dumb ecchi fanservice manga, but the author seems to need a crash course in bra size mathematics. ><;
Aimi's math actually works out (28" band, 35.5" bust = 28G or 28H), although that ribcage measurement is like, impossibly thin. But Yumi is just.... 62cm is a 24in. band. That is... toothpick. Skeletal toothpick. And, technically, with a 70cm bust that makes a difference of 3-4", which translates to a 24C or 24D (if those even exist), not AA.
I will also second the disappointment (?) of whoever pointed out that the "huge bust" girl isn't really drawn as that busty save one or two shots. It's something I've noticed very commonly in yuri manga and hentai; the characters that get the "busty" label are usually not really drawn very big at all (only a problem if you're a big bust fan like me, I guess :p).