Black women have pelvises that are more masculine than white women. The pelvis is taller and higher, and that makes the fat/muscle bunch up, giving the appearance of a larger ass when viewed in profile.
If you notice, most dark skinned black women have narrow hips, and big butts when viewed from the SIDE, not the front/back.
The extra thicc black women are all medium to light skinned and have admixture.
Similarly, Latinas only have big butts because of the North African admixture (look up tributo sangre and Canary Islands). Actual South American Natives are so scrawny they make Chinese girls look voluptuous.
The voluptuousness is the result of African and Eurasian gene mixing. Africans have narrow hips with butts that protrude in the backward direction. Eurasians have wide hips with flatter butts. Combine them and you get big hips with protruding butts, aka African Americans/Latinas.