Originally Posted by Yaatri

I meant to say the 7-'s, but the 6-'s will do too.

Thanks for your answer, but it does not address my question at all, nor does it provide any useful data. Neither 777 nor 767 existed in the 70's or 60's.
We can use Boeing 747s, which have 3-4-3 seating with a seat width of 17" as a bench mark. Their 3-43 configuration, nor seat width has not changed over the years, nor seat width.
Boeing 777 with 3-3-3 sweating have seat width of 18.5 in.
As far as I know the only airlines that operate a Boeing 777 with 3-4-3 configuration are KLM, EK and NZ. None of them is a US airline. Not all of KLM's 777 have 3-4-3 seating. NZ claims its seat width to be 17.1 in in 10 breast configuration (3-4-3) and 17.8 in in the conventional 3-3-3, while KLM claims it's 17.5 in both configuration. In any case, seat width is no less than 17.1 in, still more than the benchmark.
I don't know of any 767s with seating other 7 abreast (2-3-2) with a seat width of 17.5 in.
I don;t see any data to support that seat width has been shrinking steadily. All we can say is that some airlines reduced the seat width to what it was in 1970's, in some aircraft that had a seat width of 17.5 or 18 in.
Some of the aircraft in three airlines with seat width that is not as liberal as it is in other configurations, but still more than what it was in 1970's, impacts U.S. travellers in a big way.
The real issue is people getting fatter. Seats have not been shrinking monotonically as some people are claiming.