I feel that the issue for women in particular, but also men, is that "Lighter is better in the marathon- as long as you are healthy. enough not to get injured". Similar to women's gymnastics. Since East Africans are somewhat undernourished by Western standards growing up, they develop smaller frames than if they were to grow up in the West. This allows them to be healthy in their 20's at 90-100 lbs., with say a 15% bodyfat, whereas the Western women that are trying to get close to 90-100 lbs or less are right on the edge, maybe 10% bodyfat or even less.
It's a brutal game of physics and physiology played out over a lifetime. Much is made of the African advantage of altitude and running to school etc., but in societies where 100-lb women and 120-lb men are commonplace, you will inevitably find many fast marathoners regardless of other factors. If you are an American, you must prepare to go leaner to make up some of the "small size advantage". Rupp is also extremely lean when in competitive fetter, and the new French/Swiss guy is naturally small like the Kenyans he runs with.
I see a similar phenomenon in rock climbing, although there are no African climbers yet. Some of the top American guys are around 110-115 lbs..