They're the beachballs L&P decided to give her, part way through the story. I've played games where body modification was part of the roll-call of fetishes presented & have been choices for the player but in AWAM's case the mutation just occurred & we had to pretend she'd always looked like this. I've assumed that the Dev was catering to his schoolboy fanbase but I could be wrong. Many have probably been absent from their mother's breast for only a decade or so & subconsciously yearn for the nourishment, warmth & comfort of what seemed to be huge mammary glands to them, back then. It's the only way I can rationalise the travesty of a Dev deciding to distort his own MC in such an egregious way.
It's the most revolting, irrational development in the VN, partly because she didn't just go up a size, she grew by about three with the corresponding cup size. Now she resembles all the other top-heavy "Mom" sterotypes out there & it's unavoidable unless you stop playing. [& yes, I almost did but decided to continue out of curiousity].
Ironically, I don't dislike big breasts, as certain former girlfriends would attest. It's just that none of them grew 3 sizes while I was going out with them. That would have been disconcerting, to say the least. AWAM's "realism"....where did it go?