That because it has been said so often that fat people really do use it as an excuse without taking into considering the fact that people really do have larger bones than others. Like me and my mom for instance, her wrist bones are smaller than mine as well as her ribcage. I have a wider ribcage, but that's not because of fat. It's because of my skeletal structure. Larger ribacages are especially obvious in women with real boobs. If they have a larger ribcage their breasts tend to be further apart which make it harder to create cleavage, where someone who has a smaller ribcage, but the same size breasts, will have their boobs closer together which make them look larger and cleavage is much easier made.
Also feet and hands are another example as well as skulls and pelvic bones.
That explains a lot. Big ribcages and small boobs U___U