Looks like an A/B cup, you're right. I hadn't thought about Japanese cup sizes being different from western ones, but it only makes sense. Pixy has GOT to be at LEAST a western D-cup, more likely a DD-cup. I mean, her boobs are awkwardly flat (I think ED calls it 'pancake boob'), but they're definitely at least as big as mine, and I nearly overflow in a DD-cup bra. Admittedly she probably has a bigger band size than I do, which could change her cup size, but never in a million years would anyone look at that and think, 'Yeah, that's a nice compact A-cup there!'
It's weird. Most women, if they lie about their chest size, say they're bigger than they really are. Pixy is pretending to be somehow simultaneously 'voluptuous' and 'curvy' but still sporting a demure A-cup.
Holy cognitive dissonance, Batman!!