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).