U.S. airlines have recently grown savvier about squeezing money from every square foot of their planes. They’re shrinking legroom, pulling out lavatories, and wedging in more seats. But behind all of that, there is a more elemental strategy: Airlines are paying far more attention to how many of those seats are actually filled.

Never have domestic carriers been better at making sure their flights are packed with passengers. They’ve managed, over the last 10 years, to bring their “load factor” — percentage of filled seats — from about 73 percent to 84 percent on domestic flights, a high water mark for the industry, according government data. Some airlines, like Frontier, fly at nearly full capacity every time. That’s a major leap from earlier decades, when carriers were happy to fly at two-thirds or even half full.

The emphasis on load factor — overdue, economists say, but with aggravating consequences for travelers — is part of a drastic revenue rethink among U.S. carriers that took hold as several went through bankruptcy and mergers.

Increasing the percentage of filled seats has required a mix of obvious changes and more nuanced ones. Most simply, U.S airlines are offering fewer flights (down 16% since 2003), even as the number of passengers has risen slightly. They’ve also been ruthless about cutting out weaker-performing routes and smaller hubs.

At the same time, airlines have taken a page from the Southwest playbook, increasingly using medium-sized narrowbody jets (like the Boeing 737 or the Airbus A320) for transcontinental flights. These planes, particularly the newer models, hit the efficiency sweet spot. Smaller jets, particularly the regional 50-seaters, don’t hold enough seats for the fuel they burn. The widebody jets require more fuel and are likelier to go unfilled.

If the load factor increase looks incremental, it's caused a major change for customers. More crowded flights mean longer boarding times, less overhead storage room, and more difficulty getting rebooked after a cancelled flight.

There’s also a discomfort factor. Take Frontier, for instance, the carrier with the most significant load factor improvement over the last decade. Frontier’s A320s have 168 seats — 56 along the windows, 56 in the aisles, and 56 in the middle. Had you flown on one of those Frontier A320s in 2003, there’d have been, on average, 116 passengers aboard. Let’s assume, for the sake of this model, that all empty seats (52) were in the middle. Any passenger in a window seat would have had a 93 percent chance of sitting next to an empty chair.

Now? On a typical Frontier flight, your odds are about 25 percent.