The entire reason A321 pay is lower than 757 pay right now is because US had the largest 321 fleet prior to the merger. They had roughly 120 A321s.

APA, through its selfish endeavors didn't want the A321 flying to be paid at a rate equal to 757 flying, as that would give the US pilots an advantage in seniority list integrations, with MORE Group III positions to bring to the table. If you don't believe me, go and read thorough the seniority hearings transcripts.

The same logic applies to why E190 Captain pay is so low. Wouldn't want to closer to wide body FO pay, because that would have benefitted the US E190 guys as well.

Very classy, APA.