With catchments, dont forget the majority of north wales has MAN as the main airport.
But, the main arguement isnt really about city sizes, its about where the airport catchments lie.
For example, BHX's main issue is and always has been London airports (mainly LHR and for loco, STN/LTN). To some extent, MAN has also taken punters from the northern reaches of Birmingham, wolverhampton and stafford.
I think the case point is that in a city with one of the largest indian communities, no carrier has been able to stick out an indian connection, proven with air india, retreating to LHR. Regardless how bloomin big the city is, with London now just over an hour by train, and the ease of the motorways, LHR/LTN/STN will be always casting a shadow over BHX.
Paul Kehoe is Naive enough to think High speed rail will make BHX 'LHRs thrid ruway', more like it means more passengers have easy access to the 3 afformentioned London airports!
No doubt if these airports were harder to reach, BHX would be on traffic par with MAN, if not more.
MAN is lucky enough to be far enough away from LHR to support its own services, and our main success has been the middle east and America. This summer will see 32 flights a week to the New York area alone, and come winter the middle east will see 2x B773, 1xA388 and 1xA333, both situations envied by any other airport.
So, Birmingham may be bigger than Manchester, and if people want to see either/or as the second city, thats fine, but, for the reasons above, and without trying to sound smug, MAN will always be the bigger airport.