Hint: It has something to do with your legroom.

With jetliners filled to capacity over the summer, air travel usually involves being squished into a narrow seat with scant legroom, so the airlines can cram more fliers onto each plane, sell more tickets and make more money. Meanwhile, you’re packed in like a sardine.

Things have gotten so bad that it made news in June when American Airlines decided to cut seat pitch, a proxy for legroom, by only 1 inch on its new Boeing 737 MAX jets rather than by 2 inches, as originally planned. Talk about being thankful for small favors.

Before airlines were deregulated in 1978, seats were wider and offered more legroom. But in recent years, average seat pitch in coach has narrowed from about 35 inches to 31. On some discount carriers, such as Spirit and Frontier, pitch is as low as 28 inches. Average seat width has shrunk from 18 inches to 17 inches or less.

If that’s not bad enough, the seat shrinkage has occurred while Americans have grown larger. An average woman who weighed 140 pounds in 1960 weighed 166 pounds in 2010; the average man went from 166 to 195 pounds. 

Customers have grown angrier as airlines enjoyed soaring profits in recent years, mergers provided fewer choices, and viral videos revealed shameful treatment such as a United customer getting dragged off for refusing to give up his seat on an overbooked flight.

Seat size is usually viewed as a comfort issue, but is it also a safety concern? Flyers Rights, a passenger advocacy group, thinks so. It has petitioned the Federal Aviation Administration to regulate size, arguing…