What is the largest city in Isan, Thailand?

Isan's total population as of 2010 was 21,305,000. Forty percent of the population is concentrated in the provinces of Khorat, Ubon Ratchathani, Udon Thani and Khon Kaen, known as "Big 4 of Isan". These provinces surround the four major cities of the same names. As of 2010, their populations were: Khorat 142,169; Udon Thani 137,979; Khon Kaen 113,828; and Ubon Ratchathani 83,148.

https://en.wikipedia.org/wiki/Isan