Look, I'll agree with you on most points except the closer to flat, and Merim has a bigger ass. The fact she's got anything at all visible in her skin-tight outfit already puts her above flat. She's got handfuls everywhere, and that is much more Medium is Premium. Merim's ass is in the same class, Charlotte is the one who clears the butt arguments. Liz is the Flat is justice girl of the group (she's an AA/A-cup). Teria here has gotten a cup size boost or two from her first appearances (she's in flat is justice in the first few chapters) that put her smoothly above that.
Best I can say visually from most of the named women (some of them here like Lain the bodyguard) going from smallest to largest:
Liz, Mika, Teria, Char, Merim, Celia, Terrine.
Given the fact that we have seen Liz and Merim both with nothing on but towels. I'd probably put the other two guild receptionists next to Char and Merim
So if you want to make the category for Small instead, yeah. So she maybe more accurate to say 'small', but it isn't our fault people keep inflating medium is premium when the busts explode in size, to keep a 'scale'. Double or triple D used to be the 'big', but when we start with G or H as the High scale, even Double D seems medium.
But if we are talking butts, Teria clears Merim. the few sitting position shots we have put Teria above her.