I will say this and then drop the subject.
A 15 year old with DD tits who isn't fat is SUPPER RARE. Is it possible? Yes. If you are implying that it is common for 15 year olds to have tits this big, then I am here to tell you that you are objectively wrong. Most people in the world on average don't have big tits. America being an exception but that is more due to obesity. So I think it's unrealistic to have every teenager have big breasts. Thing is I wouldn't complain if it was just one character, but every character has the same body.
I get that this is a style choice, but I am saying for me, that style choice makes me less interested in the game. Others will like the game more with that style, and the creator has to decide who he wants to please. There is no right or wrong choice.
Had a girl in my class that was both, she was fat when puberty set in, grew huge tits, then the tits gave her confidence to slim up, turned into a huge-titted babe at 15, though her friend and mom was also big in the chest department. Her friend was always slim, but got B+ cups around the same time out of nowhere at 14, that just grew in what seemed a cup size a year. Third girl in the class didn't get a lot in the chest department, being a sporty tomboy, but got an ass more amazing in terms of proprotion to the rest of her than essentially any I've seen since outside of plastic surgery.
Yes I said third girl, because there was only three girls in my class, because I come from a pissant island (most mountainous island in the northern hemisphere compared to its size = it's a rock sticking out of the ocean) in northern norway, and there were seven students total in the class, five in the class after us (no girls), and just two students in that next class, one a girl (unsurprisingly, the school shut down a couple of years later). And that one girl two classes down had tits bigger than either one of the big-titted girls in my class, even when she was younger than them, and like the second girl she was always slim and practially elfin until she looked like a barbie girl.
That's northern Norway for you, just a few stragglers eking out an existence clinging to the side of a mountain, of course you'd have some damn good genes around then.
(Not denying it isn't super rare, just disputing the "only fat" and America specifically being an exception.)