@yarielist
She doesn't seem as far off of G as you're indicating she is. At the very least, she definitely seems solidly above the D-Cup you're considering her suitable for.
tl;dr version for the following post: Cup size alone doesn't tell you all that much about an individual's breast size; It is also necessary to know the individual's band size, before one can make any expectations as to how large or small the appearance of the breasts actually will be.
Edit: @primrosea has made a very good point, in that Japan utilizes the European labeling system, rather than that of the US.
So assuming the translators made a direct translation, her actual US cup size would be an E-cup.
Size Comparisons
Which, being that that's the smallest cup size I'd expect for her, seems like a rather solid explanation for the entire matter.
In fact, it could potentially completely explain the situation. After all, if the stat screen is based in the US measurement system (for whatever reason), it would be reporting E-Cup.
The Japanese E-Cup is equivalent to our D-Cup, so she'd be assuming it's sizing her smaller when, in fact, it's just using a different system.
/edit
It may be down to a question of familarity with bra sizing. Generally speaking, since the cup normally increases along with the band size, the result is that the slighter the frame of the individual, the smaller the breasts will be, relative to the same letter association. Moreover, the distinction between each cup size isn't overwhelmingly significant to begin with [assuming the same underlying body size], aside from the earliest cups (as a comparison against flatness tends to be much more notable). In fact, the assignation of cup sizes is close enough in sizing that simply gaining a bit of weight or preferring a looser fitting bra can be enough to change your effective cup size.
Hence why it's entirely reasonable for her to have overestimated her cup size, for reasons other than insecurity or delusion. (Rather, I'm more curious as to why she feels a vague letter association even matters.)
Read More Here
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bra size 65D (30D) and 70B (32B) will have about the same cup size. How is that even possible? If you want to have a smaller bra band you need proportionally larger cup size. So, if you wear size 70B (32B) and you want a smaller bra band, you have to enlarge the cup size, that means your new bra size will be 65C (30C)."
The first image below shows how lettering doesn't necessarily mean much [those are of equivalent effective cup size], the second image shows how a G-Cup teen of apparently larger body size has comparable breast size to the character [remember, larger body size = potentially smaller breasts relative to the associated cup size], and the third image is there to show how the difference between cup sizes isn't generally massively noticable.
Well, more specifically, differences between cup sizes aren't especially noticable when the individuals have slightly different body sizes, are wearing supportive bras [which compress breasts, potentially making cup sizes on similar body shapes appear more similar to one another], or are wearing loose-fitting clothes such as the character in question [which obscure the slight differences between similar sizes].
That all said, if she doesn't have much chest support holding her breasts back, I could see her being an E-cup. On the other side, if she's wearing a tighter binding such as a sarashi (a binding I've regularly seen associated with mikos, at least within manga and anime), her cup size may actually be much larger than it appears.