Boeing is saying that airlines are ordering more planes with more cramped economy seating than they were before, even though they're the same models of plane.
"For almost 20 years, the standard setup in the back of a Boeing 777 was nine seats per row. But last year, nearly 70% of its biggest version of the plane were delivered with 10-abreast seating, up from just 15% in 2010."
So over five years, airlines were ordering more and more of these especially cramped 777s than before, which means the same class and the same model of plane has narrower seats and aisles so they can cram in one extra person in each row.
This is a discussion on new planes, but I would be surprised if airlines weren't moving to the new configuration on older planes as well whenever they do an upgrade or refurbish (or use those planes on lower-demand routes where the lower capacity doesn't impact the actual number of seats sold).