Here is the simple answer: its just a stereotype that partially reinforces a slight gender bias in fat distrobution. The fact of the matter is that genetics decides the size of your ass and breasts. Both things that guys find arousing. You can't stop your genetics, you get what you get.
But dietary fat is allotted to the breasts and ass as well. Now what you'll find is that while any fat is split (somewhat) evenly between the two. saturated fats tend to accumulate in the ass (and stomach) more than the breasts. Polyunsaturated fats tend to accumulate preferentially in the breasts. If you eat a lot of any or both of them you'll see it depositing in both places. its just a preferential deposit, not an all or nothing.
anyway what is the point of this? If you take the most visible section of each population. So urban african americans and suburban whites, you see a distinct dietary difference. Saturated fats come from pork fat, butter, deep fried foods, etc. Polyunsaturated fats come from vegetables. It seems much more likely, using stereotypically referenced groups, for the white female to be eating lots of salad and vegetable heavy diets while the african american female tends to indulge in more of the saturated fat meals.
a person who eats a lot of fat of either group will be large everywhere.
Add in the fact that most white girls are depserate to be thin and will further cut out "fats" from their diet, missing that vegetables are a huge source of fats (albeit polyunsaturated) and you tend to get skinnier girls with less fat in the ass region and more fat in the breast region since they eat polyunsaturated fats much more now (this is compared to whatever their genetics gave them. this can't trump genes, only augment).
Black communities tend to value a little more curvy women in general, which leads to less of a push to be very thin and instead to just remain inside a healthy range. without a drive to be rail thin, the diet doesn't change dramatically... though maybe calories are watched, and the type of fats they eat cause ass accumulations.
all in all though... men report a 51-49 split with breasts usually winning out on what they prefer, but it doesn't follow race lines as much as you'd think. It is simply more visible when a black man like big ass since black men date black women more frequently, I assume, and bigger ass is more common due to the socioeconomic standards of the most visible group. While white guys love ass (I like my lady's perky butt!) but generally speaking we are stereotyped dating white suburban women whose diet causes a smaller ass but larger breasts. Its all a perception and stereotyping. Stereotypes aren't a bad thing, and they are often the most common (or else that wouldn't be the stereotype). But they are rarely the rule or even close to the rule since they simply observe trends, not reasoning.