I got it. It's because they are too perky. If she is really on her back, gravity should be trying to pull those suckers into her armpits, making them flatter around the areola area. And there is no way his tensile strength is holding the twins up like that.
I don't have friends with fake boobs but silicone might act like that since it is essentially a big fluid pocket anchored in place. Otherwise, she would have to be sitting upright for them to look that way.
Rereading this page, I realize you may have been sarcastic. My bad, if you were.
Please, why was I shocked when I saw a tiddie in chapter 0