Making a plane longer is relatively straightforward, just insert additional sections before or after the wings. Making a plane wider you may as well design a whole new aeroplane.
[1] https://en.wikipedia.org/wiki/Airbus_A320_family#Specificati...
EDIT: oh yeah found it: https://tqpr.com/airbus-new-single-aisle-airspace-cabin-ente...
> The new Airspace features include: slimmer sidewall panels for extra personal space at shoulder level
or https://www.ainonline.com/aviation-news/air-transport/2017-0...
> The Airspace package includes new sidewalls that provide one additional inch of shoulder-height cabin width
That is the most important thing imo. A 6 or 7 hour flight to your destination is so much better that the same length to Frankfurt or wherever and then waiting and taking another 1-2 hour flight somewhere. Most of the pain with flying, except maybe of ultra long-haul, is what goes on before take off and after landing. If you only have to go though that once, it's a big deal.
Plus, I dont see any material difference in comfort between modern wide and narrow body, other than your less likely to be stuck in a middle seat on a narrow plane and they board faster.