The answer is no. For a start not every Afro-American woman has the same genetic background. That said a few of the countries had, for many years, a thing where the bigger lady was considered both desirable and wealthy (it's harder to be fat when you can't eat = poor).
Not every Afro-American person, male or female, came in on slave ships. s
Some are plain old immigrants. That means, in both cases, their parents and grand parents came from a LOT of countries. Some are pre-disposed to fat more than others. Do you know where your kin originate from?
As to how an Afro-American person loses weight - the same way anyone else does.