It's also noteworthy that bra sizes as used in the game make no mention of band size, so I don't know how you're planning to get an accurate idea of what Cait's boobs look relative to Brienne or Kiyoko.
Some of the reasons for the disparity, are, in my opinion:
-The general audience likes really big boobs
-The general audience by and large doesn't know or care how bra sizes work
-More descriptive bra sizes would be very difficult to translate to an international audience as different countries use different bra sizes
-Body types in porn, especially drawn or animated porn, heavily skew towards very exaggerated and sexualised figures
-The general audience really likes really big boobs
Zag's basically right here. Frankly, dealing with band sizes
and cup sizes for these characters in an accurate way (as opposed to a "this doesn't
totally fit right, but it helps enough that I'll survive wearing it" way) would be a pain in the butt that the writers and artists probably shouldn't be responsible for given how little it actually matters. Cup sizes in this game exist mostly as shorthand for comparative booba because that's how most people in the audience understand cup sizes. Get a letter (or pair of letters, or three letters) that gets close enough and slap it into the character's appearance and move on; that's good enough for most characters, and honestly, with most characters in this game it works pretty well. The Adjatha memes mostly come out of TiTS art, not CoC2 art.
In this game, the most egregious "this doesn't totally seem right" cup size things mostly come as a result of Early DCL Concepts with characters not a hundred percent set in stone and from characters that have titty TFs where the relatively modest transformations that the writers make would end up in the art
looking like they didn't actually make significant enough changes to the character to give them really big booba. Cait's art is both of these things.