Skeletal structure only determines somebody's general size. Yes all things being equal somebody 30cm taller is going to have bigger body parts and weigh more. Even then there will always be a good handful of genetic outliers among millions let alone billions of people. It's not unthinkable that somebody's breast size can be like that. It's not like breast are attached to the skeleton like other things are anyhow. It's not any different than people having abnormally large ears, eyes, feet or any other part of the body. Also like I said before, skeletal structure basically determines ones general size. That doesn’t mean there are signals that from go from it that tells muscles to stop growing. That happens because after a while your body will get to a point where it notices the amount of muscle it has is above what it needs, and that it has an imbalance of that compared to a normal amount of fat and it will then start self cannibalizing the muscle faster than you can gain it. At that point it doesn't need to eat away at the fat when it has a lot of muscle that also weighs it down more. It's even worse when you consider the human body evolved in situations where it had to maximize energy to keep itself from starving. It will only let itself go so far out of equilibrium. That's why to continue to gain muscle you need to put them under the right type of continual stress and eat an abundance of calories. Even then there is always a limit to how far you can push it, even when you mess around with hormones. Otherwise even people on steroids could get to a size of an elephant. I guess what I'm trying to say is genetic factors and your body's biochemistry have more to do with your body's limit on muscle growth than your skeleton...
...anyway while I'm not saying that her breast indisputably real, I don't think it's totally outside the realm of possibility for somebody to have them that size even with her structure.