Here's a tough one, if fat is more valued in certain racial/ethnic cultures, then why is there this cultural trope out there that suggests black men prefer fat white women over fat black women?
Let's remove an integer - fat - from both sides of the equation, and we have "Why do black men prefer white women over black women," a stereotype that (surprise!) isn't true. But it's certainly such a meme that Chuck D satirized it in Public Enemy's acute "Pollywanacracka" nineteen years ago, and more recently Kanye in his devious 2005 hit "Gold Digger." It looks like the usual stupidity of "personal research" spread by Sherlocks who were replaced with a white woman at one point in their lives. Like being mugged by a black dude and concluding something new and misguided about black people, this stuff never holds water and people mostly just want to leave ugly experiences behind with the rationale that they at least Learned Something. Plenty of times the opposite is true. Stereotypes are steeped in fear, obviously, and in this case the fear is likely a class divide, as Kanye suggests, that successful black men by this logic consider white women to be an echelon they've ascended to. Or even uglier, Chuck D citing it as a monetary thing.
Lastly, Butch provided a citation if you care to seek it out, that deals with the "fat" aspect, namely a study proving what most people already think (hey, some stereotypes are correct!), that African-Americans are kinder to fat asses than Anglos. But she's quick to note, "overall their size preference for women was no more than a few inches larger than the preferences of white men (the study used college aged men and women, white and black, all straight, to find out what their body preferences were)." That's probably accounted for by the usual majority preferring fitter partners combined with the timid presence of vocal (re: closet) fat admirers.
The study is:
Jackson, Linda A., and Olivia D. McGill. “Body Type Preferences and Body Characteristics Associated with Attractive and Unattractive Bodies by African Americans and Anglo Americans.” Sex Roles 35.5/6 (1996): 295-307.
EDIT: A reader found the study archived online here but missing the numbers. Thanks Jon Blaze!