Comment by xandrius

I think just you can get a glimpse by what they export (from Wikipedia):

- Malawi: tobacco (55%), dried legumes (8.8%), sugar (6.7%), tea (5.7%), cotton (2%), peanuts, coffee, soy (2015 est.)

- Rwanda: Gold, tin ores, coffee, malt extract, rare earth ores

I can easily see why one has a higher GDP than the other. Very little mistery to me.