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.
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.