I thought of the comparison between Sixteen Tons and Pick A Bale Of Cotton, too, as they both seem to be an exagerated claim of strength.
The difference is that if you are doing the work because you are a black slave, rather than a very poor white abused black, at the end of the day the whites didn't have to deal with segregation and the degradation that went with being black in the South. There is an enormous difference between being owned and being free, even if the work is equally hard.

Jerry