Uh, sorry, I'm a bit confused by your first sentence. There's no girls in the game above a C/D-cup because... there just isn't?
You then say it's not an engine limitation... but then say the majority of "audience" doesn't like D/DD-cups or above. But how do you know that? What's your source on that claim? I know a lot of people I've dated have preferred my flat chest or my butt or legs, but this seems like a strange claim to make regarding fictional character bust size diversity when fictional characters are created by the creator and so he decides what they look like. What audience, exactly? Of this site, or of hentai games in general?
And I'm afraid I can't parse your second paragraph. Would you please rephrase it for me? I think it's the sentence structure that's giving me trouble comprehending your intentended information from it.