I’ve never noticed a difference in comfort between different airplanes. What matters is the seat, and they’re pretty much all the same (and, when different, this does not depend on the type of plane they’re in). Why would I care about fuselage cross section?
United has a few configurations for 777 economy seating - a 3-3-3 configuration with 18.3" of width and a 3-4-3 with 17.1" of width.
I've flown the 3-4-3 one and will never fly it again, it is super cramped, and even the few inches of extra leg room in economy plus can't make up for the narrow seats.
So being a wide/narrow body jet doesn't really relate to seat width, it's all based on how many seats the carrier can cram in.
This is entirely up to Delta though (or any airline).
Delta could have easily decided to fill Economy seats on their 737's with 18.5" wide seats... and then had a 2-and-2 configuration instead of 3-and-2 or 3-and-3.
Same is true for the 777, they could go with smaller seats and pack in more seats per row. Likely, the 777 is used for longer flights and they decided a more comfortable seat for an 8-16 hour flight is worth a few less passengers. Or, the 777 might be overweight if filled with too many passengers, so larger seats are justified.
At the end of the day, it's the economics of making these flights - airlines run on notoriously thin margins, and flying a passenger jet is very expensive.
That makes no sense. Seat width is fuselage width divided by number of seats, minus room for the aisle(s). Wider seats means fewer seats. The airlines make that trade off based on what they think people will tolerate.
