Steatopygia is a high degree of fat accumulation in and around the buttocks. The deposit of fat is not confined to the gluteal regions, but extends to the outside and front of the thighs, forming a thick layer reaching sometimes to the knee.

Congo

It's called steatopygia.

Perhaps she has an extreme and excessive accumulation of fat in the posterior region, a condition known as steatopygia.

Steatopygia. noun. [derived from Latin, steato, "fat" + pug_ "rump"]. A protuberance of the buttocks, due to accumulation of fat in and behind the hips and thighs.