Jet airplanes may be able to defy gravity, but even the most powerful craft can’t escape the cruel physics that govern La Guardia Airport.

Squeezed onto a peninsula at the edge of Queens, the 67-year-old airport has long been among the most congested and constrained in the country. Its two 7,000-foot runways can handle a maximum of around 75 planes an hour, or about one every 48 seconds, according to the Federal Aviation Administration.

That doesn’t come close to meeting demand, and authorities say there is little hope of improvement. La Guardia is out of space, making expansion impossible.

Still, the Port Authority of New York and New Jersey believes it may be possible to move an additional 8 million passengers a year through the airport, and the FAA is considering a range of new regulations aimed at using every ounce of its capacity.

The secret, both agencies say, is pressuring airlines to fly bigger planes.

In recent years, the aircraft using La Guardia have been shrinking, even as demand for seats has increased.

On one typical day in early October, some 600 of the airport’s 1,194 scheduled flights were on aircraft with fewer than 70 seats, the Port Authority said. Overall, the average number of seats per departure has fallen from nearly 110 in 2000 to about 97 last year.

Airlines say the change benefited travelers by allowing carriers to fly popular routes several times a day, without having to fly part-empty. But it has also frustrated aviation officials, who would rather see passengers combined onto fewer flights.

“We need to move to larger aircraft,” a Port Authority spokesman, Pasquale DiFulco, said. “Customers want more choice of flights … and that’s what airlines provide. But there is no question that, to some extent, that stands in the way of our desire to meet increased demand.”

Since the summer, the FAA and Port Authority have been discussing competing proposals that would each make La Guardia the first U.S. airport to threaten airlines with revocation of precious flight slots if they don’t fly bigger jets.

The FAA’s plan would require most airlines to meet an average aircraft-size target, probably of between 105 and 122 seats per flight.

The Port Authority favors setting minimum plane sizes on a gate-bygate basis, based on how many passengers each can handle. Both plans have run into opposition.

The Air Transport Association, which represents the major U.S. airlines, formally objected to the FAA’s proposal this month, calling it “governmental micromanagement.”

A spokesman for the association warned that service to smaller destinations from La Guardia could be compromised if the airlines are forced to fly bigger planes.

Simultaneously, the FAA has proposed tackling the sticky issue of encouraging more competition for scarce La Guardia flight slots.

For nearly four decades, flights at the airport were controlled by a rationing system that limited congestion, but also made it nearly impossible for new air carriers to get access to the gates.

Congress decided in the spring of 2000 to encourage competition by decreeing that the old rationing system for La Guardia and other high-density airports would expire by 2007. It also ordered transportation officials to issue new flight slots to airlines that had been shut out.