The airlines are doing their best to encourage/create space encroachment with the shrink set space:
https://www.wsj.com/articles/the-best-and-worst-coach-seats-on-u-s-airlines-1497452020
"Some airlines are down to 28 inches of space from the back of your seat to the seat back in front of you in their basic coach rows, down from the once-standard 32 inches of what’s called seat pitch. That’s a difference felt right in the kneecap. Airlines considered full-service have shrunk down to 30 inches and American Airlines, which 15 years ago gave each row of coach seats 34 inches of space, considered going to rows with as few as 29 inches."
"Take 3 inches out of padding out of a seat back and you can push the seats several inches closer together without reducing personal space. In many cases, the new seats are more comfortable."
"Which airline has the best coach seats?
JetBlue . Rows are 32 inches long, planes have fast Wi-Fi service and seats have free seat-back entertainment. “Their Airbus aircraft consistently score higher than anyone else,” says SeatGuru’s Mr. Counter.
Virgin America is close with 32-inch seat pitch and similar amenities. Southwest has some planes at 32 and others at 31. These three so-called value carriers, which often don’t have the lowest prices but build a strong following with their comfort and service, offer the most legroom.
Which have the worst?
Spirit and Frontier are two major U.S. airlines with 28-inch seat pitch on their fleets. Even renowned cheapskate Ryanair has more. Around the world, only charter operators are typically that tight. SeatGuru says Spirit is “far and away” the most poorly rated in reviews for U.S. travelers.
What about the Big Guys?
American, Delta and United have standardized most of their planes at 30 to 31 inches of seat pitch. Adding rows creates capacity growth without the expense of new airplanes and makes a low-margin business more profitable.
Widest and skinniest seats?
Boeing single-aisle planes typically have 17.2-inch-wide seats in coach. Airbus single-aisle planes have 18.3 inches because of a wider fuselage. The newest seats coming from manufacturers actually are built to hug the sidewall more closely, moving the window seat closer to the window. That allows for a seat one-half inch wider on 737s, measured inside the armrests.
Seats on widebody planes used to be 18 inches wide, but now that is dwindling to 17 inches. The Boeing 787 was designed to have eight seats across, but now almost every airline flies it with nine 17-inch-wide seats in each row. The Boeing 777 has gone from nine abreast to 10. One airline—French leisure line Air Caraïbes, has installed 10 seats across each row of its brand new Airbus A350 jets. Other airlines have nine seats, but will that last?
“The difference between 17 and 18 is a big deal. You lose wiggle room. You’re shoulder to shoulder,” Mr. Counter says."