The 800 is an interesting distance, it's the "middle distance" of running. Shorter races are "sprints", longer races are "endurance", but the 800 is right in the middle. As such, having more mass would be beneficial for the sprinting aspect - but it would hinder someone in longer distances as it's extra weight to carry around (you don't typically see larger people winning marathons).
The top 3 finishers in this race in question are all African - and the 800m races have historically been won by Africans (also typically true in the men's race). In shorter races, both male and female, you see more Americans, those from the Caribbean area, and Europeans. In longer races (1500m, 5k, and up) you see people from all over the world, but typically much smaller in stature - for instance the man that won the Olympic 5k is 127 pounds, while the winner of this race, Caster Semenya, is 161 pounds. So the extra mass would be a disadvantage there.