Two days ago I flew with KLM from Calgary to Germany, first with the "Dreamliner" 787 and then with the 737 in economy class. I had the impression (due to my impaired space during eating on the 787) that the seats in the 787 are narrower than in the 737.
Is this true or just my imagination? With the distance between the seat rows I am not so sure. Maybe reduced too?
Turning my comment to JonathanReez into an answer, not to contradict him to to offer an explanation as to why his answer might be correct while also your experience of the seat being smaller might still be true.
The seat space depends on how you measure it - a 17.5" seat cushion or between the arm rests may still leave you with less space overall because airlines have started shrinking arm rests and aisles in order to start fitting 9-abreast, 10-abreast and, god forbid, 11-abreast seating into their aircraft.
The outcome of this is that you may still get 17.5" of width in which to put your bum, but your shoulders and elbows have less room and thus the overall space feels more cramped.
This is something you will only see in widebodied aircraft, because a 737s 6-abreast is currently the maximum an airline is legally allowed to use, resulting in a 3-3 configuration. On widebodied aircraft, the standards have risen from 2-2-2 to 2-4-2, 3-3-3 and we are starting to see 3-4-3 being offered, and the space for that increase comes generally from the aisles and seats themselves (although some airlines are finding a few inches from narrower fuselage linings).
According to SeatGuru, KLM's only 787 configuration has seats with 31" pitch and 17.5" width. On the other hand, all 737 configurations have 30" pitch and 17" width.
So the answer is: no, the 787 seats are not narrower. The difference might have been purely psychological as the 737 flight was a short hop, while the 787 was a cross-continental trip.
