Smaller lavatories are helping airlines to add extra seats to new and existing aircraft for more profit, but some passengers — if they can get into the bathrooms — say they are being shortchanged.

The continuing installation of smaller and reconfigured bathrooms, which began in late 2013, has led to complaints about safety issues, say travelers and flight crew, who are concerned about restricted access for the physically disabled, as well as ease of use for other passengers

Barry Brandes, a retired singer from Somers, N.Y., travels several times a year on United Airlines. At 6-foot-4, Mr. Brandes said that getting into the new lavatories on the Boeing 737-900, a single-aisle airplane, is not easy. “If I don’t duck, I hit my head on the door,” he said. “I can’t stand up completely, so I have to twist myself into a pretzel to use the facility.”

United has a total of 250 Boeing 737-800 and 737-900 aircraft that feature a combination of the new lavatory and the traditional lavatory, according to Erin Benson, a spokeswoman. In an email, she said the airplanes are reconfigured for the best use of space. In some aircraft, she noted, lavatories in first class have a new design, but have not decreased in size, while new lavatories located between first class and the first row of economy have decreased in size.