District levels

When I looked around on the website of the Surat Thani provincial office of the Department of Provincial Administration (DOPA), one post in the forum caught my attention. Posted in January already and not receiving any answer by the webmaster the question reads

ไม่ทราบว่าหลักเกณฑ์การแบ่งเขตอำเภอของจังหวัด สุราษฎร์ว่าเป็นอำเภอขนาดเล็ก กลาง ใหญ่
หน่วยงานไหนเป็นผู้รับผิดชอบ ใครทราบช่วยบอกหน่อย เพราะต้องการข้อมูลทำวิทยานิพนธ์ด่วย

If I understand it correctly, the poster asks for the rules of the categorization of districts into small, medium and large, and which authority is responsible for this.

Last year I posted about district levels already, however at that time I found five levels, which seemed to be about the economic strength as well as population of the district. Trying to find more on the above mentioned three levels, I could find at DOPA a list of award winning district offices in the three categories - each with several photos of the inside of the district office. The one of Phachi district, Ayutthaya province, even includes a photo of the award badge.

At the website of Nakhon Pathom provincial office of local administration a news report lists the districts of this province with their category.

อำเภอขนาดใหญ่	Large	Mueang Nakhon Pathom, Sam Phran
อำเภอขนาดกลาง	Middle	Nakhon Chaisi, Kamphaeng Saen, Bang Len
อำเภอขนาดเล็ก	Small	Don Tum, Phutthamonthon

Now, since there were the district office awards issued according to these three levels, I checked the report I wrote on this award last year. It also had three district levels then, and also listed the population numbers which qualify the three levels. Just sadly this does not fit with the district of Nakhon Pathom - as the levels change at 50,000 and 80,000, the three middle class districts should be at the highest level as well.