I am sure the airlines configure their planes for the "average" persons comfort. The average male is 5'8", and wears the equivilant of a womans size 14. The average female is 5'4", and wears the equivalent of a womans size 12. From first hand experience, the average person will fit into an airplane seat with a reasonable amount of comfort, at least for a short haul flight.
Regretfully in order to make the seats wider, they would have to either pull out an entire seat in each row (watch your airfares SKYROCKET!) or buy all new planes. Neither is going to happen.
I like the idea of having a few rows on each flight with first class size seats, but not the other first class amenities, which would be vailable for purchase at a surcharged amount. (ie, two seats where three would be at 150% of the normal fare each--and this could also be the normal discounted fare, not just unrestricted.) This might solve part of the issue. First class is usually about three times the unrestricted coach fare, btw. I do want to say that first class seats are still not large enough to comfortably seat a person in the over 300 lb range, and the arm rests on them don't go up, so first is not at all the best way to go for some people.
Anne