Not necessarily muscle size
I trained with many elite runners from Africa in my days. From Kenya, Morocco, Ethiopia, Eritrea, Somalia. Some were world class, some were just very good. Some of us Euro types were just as good. But there was never any disagreement that the Africans were a "different breed". I say that with respect and admiration. Typically higher set calves, almost always flat feet (that did not pronate), longer arm, small torso, narrower hips, generally more ectomorphic. Also they may have had the same thickness of thigh, but their knees protruded more, so there was less muscle mass around the knee. Anyhow, the differences were obvious and I find it unbelievable that people cant appreciate and acknowledge these differences. I do believe these characteristics are what makes the difference.
BTW check out the builds of both Rupp and Mo. While they are both fantastic athletes, they have the subtle differences I'm talking about