Interesting hypothesis regarding relative numbers of records in genres.
I tested it with my Ohio collection which is organized by rockabilly/early rock - including later records such as the two posted above, garage and rock from 1964 until 1970-ish, soul from early 60s until mid 70s (give or take a few), 'straight' country (anything that popular opinion would consider rockabilly is in the first section) and R&B/doowop. All these sub collections include the biggest hits down to the unissued acetates.
The garage/rock collection fits into 5 of the standard 45 boxes, this collection is 99% complete vs known records. The early rock/rockabilly collection, about 90%* complete, fits into 4 standard 45 boxes. The soul collection, about 85%* complete, fits into 4-5 boxes. In other words, the number of released records is roughly the same, with the soul collection leaning as the largest. The soul collection includes probably every Edwin Starr and O'Jays 45 so even large hitmakers that the general audience doesn't associate with Ohio are in there.
Obviously, Ohio is not Detroit or Chicago when it comes to obscure soul. I'm sure there thousands of soul records from both cities. Ohio is a good cross section when it comes to demographics in the US during this time, as the population decline and rust belt rot was just starting in the time period I cover.
FYI for discussion.
*these nunbers may be low, in fact, when I think of the records I am after, only maybe 10 records come to mind immediately.