Originally Posted by brunos

Boeing has been able to squeeze one more seat abreast on its 777 and 787 making it less comfortable for the pax compared to A330 or A350. But that is a benefit for the airline.

Airbus had launched a big advertising campaign stressing the fact.
A350 cabin is wider than B787 cabin (5.58m compared to 5.4m), so it is natural to fit 9 abreast on A350 with a seat width of 18" compared to 17" on dreamliner which was initially marketed with a 8 abreast config in Y.