Regardless of population figures, (and if we're going to be pedantic, the City of Manchester, i.e. the local authority, is the 7th largest in England), MAN is still the busiest hub airport outside of the Southeast.
Therefore AE's point about it being strange that only one of the three NI airports will have services to it as of Jan 10th, is a valid one in my mind.
Agreed, the point is indeed valid. There is definitely an opportunely for someone to add capacity between NI and MAN.