I think it's because we're so used to seeing white girls hide their thickness and starve themselves. So, now that hips and ass is in, white women aren't killing themselves to stay thin. For so long, if a white chick had some meat on her bones, she wasn't considered atractive. Let's face it, for so many years the white boy's standard of beauty was based around his fragile ego. Thinner than him. Shorter than him. And as docile as possible. Basically, she had to be built like a pre-teen boy with tits.
That translates into us thinking that white chicks just don't naturally have bodies like that. While, in actuality, a good number of WW have had them all along. It was just hidden through starvation and clothing designed to camouflage said ... assets.