When I compared the 2006 and 2009 updates to Taagepera’s lists, I missed one new empire in the 2009 article: Scythia. I have added it to the original post.
I said that the 2006 article added eight empires to Taagepera’s lists. I was wrong. I trusted a note on page 221 of the 2006 article by Turchin, Adams, and Hall:
Our list of large historical states was based on the compilation by Taagepera, which has been systematized and posted on the web by Chase-Dunn and coworkers http://irows.ucr.edu/. We checked the Taagepera list with all major historical atlases in the library of the University of Connecticut and found eight additional empires that fit our criteria (Axum, Hsi-Hsia, Kara-Khitai, Srivijaya, Maurian, Kushan, Gupta, and Maratha).
Four of their eight empires (Axum/Aksum in the Horn of Africa, Srivijaya in Indonesia, the Hsi-Hsia/Western Xia who were rivals of the Song Dynasty in China, and Maratha in South Asia) appear to be absent from Taagepera’s articles, but the other four are present and accounted for: Kara-Khitai (as W. Liao in Taagepera 1997), Gupta (Taagepera 1979 p. 132), Kushan (Taagepera 1979 p. 132), Maurian (as Maurya in Taagepera 1979 p. 132).
After a chat with T. Greer of The Scholar’s Stage, I read an interesting article by Peter Turchin called “A theory for formation of large empires” (2009). He is curious whether other world regions show the same pattern as China of empires beginning in the steppe or in the neighbouring farmland not the richest and safest agricultural districts. As he says, a lot of research focuses on the decline and disintegration of empires, not so much how a single king can come to rule millions or tens of millions of people in the first place: why do some empires last centuries when most fall to pieces within decades?
Turchin catalogued 64 states until the year 1800 CE with an area of at least a million square kilometers, and found that “over 90% of historical mega-empires were located next to or within the Old World arid zone extending from the Sahara desert to the Gobi desert” (which is a slightly different claim than the one about steppe frontiers, but never mind). When I read his list, one line popped out at me:
The table lists a Median empire with 2.8 million square kilometers in -585 (which is 586 BCE in Julian astronomical years with a year 0, but I think he means 585 BCE). That would have been as large as Iran, Afghanistan, and Pakistan combined. And the trouble is that such an empire probably did not exist, and if it did exist we don’t know its area.