A slam pig is like a cheat meal/day, it's extremely common; it's doesn't really fall along race lines.
If we really want to divide it on race lines it could be some ancient instinct to have a fat woman means you have a secure life on the African plains, often fat woman are desperate and hate their dads (as most women do and find dating a black man as a step up since most white men wouldn't give them the time of day) and many black men use the fat white woman (really just the fat woman in general) as a stepping stone to get their life back on track.