The scope to which they are regulated depends on a host of considerations, most notably the potential emission levels generating by company operations.
Absolutely agree
(e) The department normally does not require public hearings in connection with applications for minor projects, as identified in sections 621.3 and 621.4 of this Part. If a public hearing is required for a minor project, the application shall be treated as a major project for purposes of this Part."
Still with you. Here is where I turn:
State facility permits are issued to facilities that are not considered to be major (as defined in the department's regulations), but that meet the criteria of Subpart 201-5. These are generally large facilities with the following characteristics: ... They require the use of permit conditions to limit emissions below thresholds that would make them subject to certain state or federal requirements ...
So I see it as a minor project being treated as major by virtue of meeting criteria in Subpart 201-5. That is met by virtue of the CO, NOx, and SO2 emissions (at minimum) that are defined as criteria pollutants and therefore regulated air pollutants.