So you don't hit the ceiling.

WSJ: Ringling Brothers' Air Apparent

On a recent trip, passengers scooted sideways down a 16-inch aisle, some slumped over so their shoulders didn't hit the small overhead bins and their heads didn't scrape the ceiling.

WSJ: Regional Jets: Are These Model Airplanes?

So the theory has always been that China couldn't really - they would hit a ceiling, at some point.

NPR: Role of U.S. Changing in New World Order