Now, there are definitely more common traits of specific races, but I disagree about Africans and Islanders putting on muscle better than other races. I've never heard of this. If this were so, Africans would completely dominate all sports. Overall, it seems a pretty level playing field. It's more the individual's genetics that determine how easily they can put on muscle.
People are considering this racist, because it's stereotypical of black women to have big butts. Now, the big breasts thing isn't actually part of that stereotype. I think it's obvious she is just trying to imitate Serena. Apparently they are good friends, and this is just something she thought would be fun. The thing is the stereotype of black women having big butts doesn't mean women from other races don't have big butts. We all know we've seen women from all races with big butts. It was definitely individual, nothing to do with race.