So no, the A333 is not 10% smaller than the 789, no matter how much you want it to be. This we can see based on how airlines configure their aircraft at the moment. I doubt the A332 will be 10% smaller than the 788 seeing that is more than 2m longer, or about 3 rows of Y, but we don't have a comparison of airline configured cabins as we do with the 789 and A333.
Avianca, American, China Southern, Qatar, Hainan, Oman operate both, the A330-200 and the 787-8, although Business seating can be quite different and Royal Jordanian and Jetstar did operate both as well.
332 20C 12Y+ 226Y (238Y)
788 28C 55Y+ 143Y (198Y)
as 40Y seats is more than 8C seats, A332 is "bigger"
332 30C 222Y
788 28C 222Y
different C class seats, might be advantage 788 here
332 v1 4F 24C 48Y+ 142Y
332 v2 24C 50Y+ 184Y
788 4F 24C 200Y
call this a draw, as 6 rows Y+ cost about one row Y
332 v1 24C 248Y
332 v2 24C 236Y (flatbed C)
788 22C 232Y
close to equal
332 36C 186Y
788 36C 177Y
probably the best comparison here! Both use roughly the same seat.
332 30C 196Y (new Business)
788 18C 249Y (new Business)
Oman Air has an immensely big Business Class seat. 12 Business class seats would take out 7 rows Economy according to their seat maps. That equals 63 seats and would bring the 788 to
potential 788 30C 186Y (or 195Y, if you assume only 6 rows of Y removed)
Overall though, for roughly the same seat amouts or slightly more onboard the A332, the A330 offers a serious comfort advantage. I personally tried both and find it impossible sitting in 3-4-3 (777) or 3-3-3 (787) for more than 3 or 4 hours. Thats actually my idea of hell.
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