For a long while, the "small jet" line in the sand was 50 seats. Then, 70 seats were surrendered.
Then came post-9/11 bankruptcy for the legacies where scope was partially sold, partially taken. If you throw out the unique United/Air Wisconsin B146 situation, Airways allowing CRJ-900s was the first big hit to scope but it wasn't until Delta allowed 76 seaters that things REALLY took off.
DALPA's post-bankruptcy contracts have become the defacto "industry standard" with regard to scope; you can see the impact that had on the recently ratified 1113 contract for AA and the UAL/CAL TA currently under vote.
Basically, 76 seats is now the industry-standard "line in the sand" regarding "large small jet" scope, with the exception of Airways flying some 86 (I think?) seat CR9s and E175s.
The DALPA contract, APA contract as well as the UAL/CAL TA tie the number of "large small jets" to mainline fleet size. In order to add large small jets, the number of mainline planes has to go up.
Of course, throw in another force majeure event like 9/11 and the scope restrictions could go out the window...