With respect to economy seats there are two distinct situations:The problem is that the seats keep getting smaller and the people keep getting bigger. DH is a big guy, not obese but 6’2” and broad shoulders. On one flight he was stuck in a middle seat between two obese people. I was a few rows away. Had I realized his situation I would have traded with him since I was in a row with two average size people. This was a flight where we were unable to check in early and I was put on standby even though the flight had been booked months in advance.
Even with average size people that middle seat is not comfortable. I usually end up in the middle seat since I book the aisle for DH and the window for me in hopes that the middle seat stays empty. It usually is not empty and I offer to trade with the middle seat person.
Wide body aircraft: These are aircraft with two aisles like the Boeing 777 and 787, Airbus A330, A350 etc. Seat width has actually decreased in such aircraft. The Boeing 787 for example was intended to have a 2-4-2 seating configuration but most airlines have switched to a 3-3-3 configuration with narrower seats. On the other hand, leg room is typically more generous with 32 inch seat pitch for long-haul aircraft.
Narrow body aircraft: single aisle aircraft such as the Boeing 737 and the Airbus A320 have the opposite problem. Seath width has not changed in decades. These aircraft were designed with a 3-3 seating plan which has not changed. However, seat pitch has greatly decreased over the years leading to very poor legroom. I have seen seat pitches as low as 28 inches on budget carriers.
Ultimately, most people want the cheapest possible fares and aren't willing to pay for comfort. Some years ago, American Airlines increased the seats pitch in economy by reducing the number of rows. But they lost customers to other carriers who had lower fares. They learned their lesson and no carrier has repeated this experiment.