More passengers, fewer planes. That's the mantra of the modern airline, which is constantly working on increasing efficiency in order to increase profits.

United Airlines announced on Thursday that it expects capacity growth of 1.5% to 2.5% on its entire fleet, with an increase of .5% to 1.5% on its domestic fleet. Part of that growth will be new planes, but part of it will be made on existing ones.

"The major driver of growth is the rollout of slim-line seats," said United's vice president and chief revenue officer Jim Compton during the airline's earnings call on Thursday. "We currently have installed these seats on more than 300 aircraft ... the impact of higher utilization will represent nearly 14 additional aircraft becoming part of our fleet without spending any additional capital."

That's 14 new planes nearly created out of thin air.

Slim-line seats are intended to add capacity without further crushing economy passengers, and a Boeing spokesperson said that the seats are “giving passengers more leg room than the previous generation of seat designs."

But everyone is noticing that seats are getting smaller, and not in a good way.

Headed back to DC from Phx - are you as frustrated as I am that the airlines keep moving the rows of seats closer and closer together? — John McCain (@SenJohnMcCain) January 13, 2014

According to an internal survey by United last year, passenger ratings of slim-line seats improve over time.

That could be because they get worn in and become more comfortable, or it could be because passengers get used to it.

Decreasing seat pitch — the distance between the same point on two consecutive seats — was once just the territory of budget airlines like Ryanair and Spirit Airlines.

However, the legacy airlines are following the trend, and an economy seat that used to have a seat pitch of 32 to 34 inches now typically has 31 inches. United isn't the only airline making more room on existing planes with smaller seats.

According to data from SeatGuru, the roomiest economy seats on the four biggest airlines are all smaller than the smallest seats were in the 1990s.

It might not be all bad news for passengers, though. In a recent survey by YouGov, 42% of respondents said they would inclined to a purchase a seat with less legroom if it's cheaper.

In the earnings call, Compton said that the planes with higher capacity also add to the airline's revenue from extra fees.