It depends on where the McDonald's is located. You won't get a McDonald's filled with fat black folk if you go out to bumfuck Farmville in the Midwest. The Walmart and adjacent McDonald's near me are probably 85% black (both customers AND staff), but I've been to numerous Walmarts and McD's in places like Kansas, Wyoming, South Dakota, etc., that never had a black person in sight. While you might see a lot of fat black people where you are (near a major metropolitan city, most likely), like Soygen said, the TRUE fatties in America are the upper-middle class white folk in suburbia who drive everywhere and sit in office chairs 8+ hours a day. And there are A LOT more of them than there are inner city fat black people.
In other news, I destroyed a Ruby Tuesday salad bar the other night. Why are salad bars so rare nowadays? You used to see them all over the place, but now it seems like only Ruby Tuesday and the occasional buffet (Golden Corral, etc.) have them.