I know AA and NW have more F seats on some planes, but remember that UA has some massive domestic widebodies with tonnes of F seats. On average, maybe AA and NW have more per plane, but not that much more.

Also, what matters is not the number of seats, but the upgrade percentage. From what I read here, the upgrade percentage seems about the same on most US carriers.

Also, NW F sucks. 34" pitch!