The result is upset travelers, harried flight attendants and delays.
The percentage of passengers bringing bags on board has hovered around 87 percent in recent years, United Continental says. And "the size of the carry-on has increased. ... They are stretching the limits of their bags," says Scott O'Leary, managing director of customer solutions at United Continental Holdings Inc.
Expanding bins is a competitive move, says Henry Harteveldt, who leads airline and travel analysis at Atmosphere Research Group, a market research firm. "Especially if they cater to the business traveler, they're hoping it will give them a small but noticeable competitive advantage."
At first blush, it might seem like airlines risk giving away fees if more people can fit carry-ons on board. But they're not risking much as it turns out.
Airlines often waive bag fees when luggage can't fit overhead and must be checked at the gate. And business travelers, who generate most of the industry's revenue, are often exempt from baggage fees anyway.
But will bigger bins encourage fliers to bring larger bags? Airlines hope not, and are trying to crack down before luggage makes it into the cabin.
Aside from installing larger bins as United Airways Group, Delta Air Lines and American Airlines have done on some planes another option is to install new doors on the overhead bins. The new doors curve out more than the old ones, allowing passengers to slide bags into the compartment wheels first instead of sideways.
United and American are in process of changing bin doors on some airplanes.
Boeing wants its new planes to have the right bins for all that stuff.
The company is engineering its bins to be a better fit for a standard 9 x 14 x 22-inch roll-aboard bag. That's a change from the past. Designers used to focus on maximizing cubic inches. That produced impressive-sounding space that would be quoted in Boeing's sales materials. But it wasn't necessarily a good fit for actual carry-on luggage, says Kent Craver, Boeing's cabin expert.
"We never used to talk about how many bags would fit. We talked about volume," he says.
In designing bins on its new 787, Boeing dispatched workers to Costco and other stores to buy roll-aboard bags to make sure they would fit.
The extra space also makes it more likely that bags will end up close to the passenger who brought them.
"They don't want it 20 rows behind them or 20 rows in front of them, because that causes a lot of anxiety," Craver says.
Bigger bins help. So would passengers who follow the rules about carry-on sizes.
American is trying to be tougher about carry-on sizes. At every gate, it has installed new size-checking boxes with three hard sides. Bags either slide in or they don't.
The old checkers had no walls, so it was easier to fudge. Airline spokesman Tim Smith says the new boxes act as an arbiter when customers deny that their carry-on is too bulky.
American will charge passengers to check an oversize bag at the gate.