There was once a time not too long ago, when AA had more flights out of Chicago than UA, despite it being UA HQ.
When was that, and do you have more details?
I don't recall comparisons of revenue passenger miles or available seat miles, but this shows seats for the summer '13 schedule on a typical weekday. (Pre-merger; no US)
Hub..........Seats......Flts.......Avg...Mainline. ..Express
ATL/DL....132300....1026.....129.0.....705.....321
DFW/AA....90500.....807.....112.1.....504.....303
CLT/US......68100.....655.....103.9.....286.....369
IAH/UA......58800......618.....95.1.....253.....365
ORD/UA.....56700.....659.....86.0 .....199.....460
MSP/DL.....49700.....479.....103.7.....195.....284
DTW/DL.....49300.....542.....91.0.....174.....368
MIA/AA......46000.....319.....144.0.....251.....68
EWR/UA....45800.....429....106.7.....203.....226
ORD/AA.....42700.....505.....84.5 .....169.....336
PHL/US......41000.....467.....87.7.....164.....303
MDW/WN...38800.....265....146.3
http://www.airliners.net/aviation-fo....main/5914878/