The number of available seats is irrelevant, what matters for any specific route is the available bums to put on the seats.
Economy is driven by occupancy, running a small aircraft full of passengers will always make more money than a larger aircraft with too many empty seats.
The market is evolving towards tthe centre, small aircraft are getting bigger to cope with more traffic and at the same time large aircraft are getting smaller to cope with more secondary destinations and less big hubs.