An interesting comparison is against the newer pre-merger AA A321 configuration, which is:
16 first class
36 main cabin extra
129 economy
So Delta is somehow managing to put 172 coach seats in compared to AA's 165, even though Delta has an extra row of F.
For one thing, AA maintains a proper bulkhead between F and Y as well as a reasonable amount of legroom in the first row of each cabin. Those decisions account for some of the "wasted" space in the AA configuration.