I've known several women who were legitimately this sized. I'm not going to lie and say they were all happy about it, but when they were feeling strong and comfortable, they enjoyed what only they could do with breasts that size. That being said I've met some who were considering enlargements on top of this size... definitely an exception but it's quite heartening nonetheless.
One of the things that I believe helps is that many of the little things that the characters say and how they behave stem from these girls stories, as well as some important ones of my own (Even if I am lacking quite that level of endowment). So you are right in so many ways, and the hassle of having to lean forward in every hug is definitely felt, and the harsh gaze of their entire worlds is a shaping event for every character. I certainly have tried to express not a game about big breasts, but a game about characters with big breasts, even if their spines are on a bit of the fantastical side.
I'm sad that they are a little too large for your immersion and I appreciate the points back for story. At the end of the day though it is your immersion and other peoples opinions shouldn't devalue that. To me, your feedback makes me a better story teller.