Very interesting! These findings are consistent with what we find in 3ie's impact evaluation repository, which is not all economics, but is all development. See numbers reported in our blog post below. I can't seem to copy in picture of the heat map, but there is one in the post. Full-length article coming soon.
"The top 10 countries are India (390), China (281), Mexico (247), Kenya (233), Bangladesh (197), South Africa 194), Brazil (193), Uganda (173), Pakistan (105), and Peru (105). For some countries, the number of impact evaluations has more than doubled from what we found in IER 2012. These include Brazil, Cambodia, Cameroon, Costa Rica, Malawi and Mozambique. We also found impact evaluations for a few countries that did not have any in IER 2012. These are Antigua and Barbuda, Dominica, Saint Lucia, Somalia and Yemen. Meanwhile, we continue to notice a lack of impact evaluations in regions like Central Asia and Central Africa." http://blogs.3ieimpact.org/is-impact-evaluation-still-on-the-rise/
We explore the geography of development impact evaluations further, based on an earlier version of the IER, in this open access article: http://www.tandfonline.com/doi/pdf/10.1080/19439342.2015.1034156
Why does China feature more prominently in development impact evaluations than in development economics? There is a large number of public health studies conducted in China.
The population hypothesis is relevant generally, but does not explain the geographic distribution of impact evaluations of peacebuilding interventions. http://www.3ieimpact.org/en/publications/3ie-evidence-gap-map-report-series/3ie-evidence-gap-map-report-1/ We found in the peacebuilding evidence gap map that Liberia has the most studies. I call that the "Blattman effect" :-)