Airbus officials say they don’t promote the configuration for intercontinental flights. “On long haul, we believe it needs to be a minimum of 18 inches,” said Airbus’s Mr. Emerson. “If it’s a regional flight, we can accommodate one more seat abreast.”

Mr. Emerson said seats on an Airbus superjumbo at 11 abreast are about the same width as a Boeing 777 at 10 abreast.

But the packed A380 would have an extra drawback because the center bank of seats, between the two aisles, would be five abreast: In every row, both of the window seats and the center seat would be two seats from the aisle, an arrangement known to frequent fliers as the ‘double excuse-me.’ That means three passengers in each row could face “this horrendous position of having to cross two people to reach the aisle,” said Mr. Wong at Trip Advisor.

Airbus, like Boeing, is making high-density seating easier. On the new Airbus A350 model, now in development, the company is proposing either nine-abreast seating or 10-abreast.

Airbus dubbed the model “XWB” for extra-wide body, because the A350 is wider than its A330 and Boeing’s Dreamliner. But Boeing officials note that Airbus is proposing to put the same number of passengers in each row of an A350 as Boeing traditionally put in rows of a 747 jumbo jet—inside a cabin roughly 21 inches narrower.