Is the passenger capacity of the A350 really only 250 pax?
Airbus themselves give max passenger seating 440, and typical seating 3-class 300-350, for the A350-900. https://www.airbus.com/aircraft/passenger-aircraft/a350xwb-family/a350-900.html

The undeveloped A350-800 is listed with 276 passengers. Maybe that's where you got your 250 number.