Christoper Caldwell's Age of Entitlement describes a NYT paper from 1987 that was 1,612 pages and weighed no less than 12 pounds. Why was there a 1,612 page newspaper? How did this lead to the ~50 page papers of today?
I'm assuming you're referring to the September 13, 1987 (Sunday) paper, not the September 14th (Monday) paper.
As archived in Proquest, Sept. 13 was 1,158 pages, but no single subscriber received every regional section.
It was broken up like this:
Main: 92 pages
Arts & Leisure: 52 pages
"The Guide" (the upcoming week in TV, film and dance): 24 pages
Business & Finance: 64 pages
Week in Review; 36 pages
Sports: 38 pages
Magazine Part 1: 164 pages
Magazine Part 2: 88 pages
Book Review: 64 pages
Real Estate: 88 pages
Want Ads: 64 pages
Travel: 42 pages
New Jersey Weekly: 32 pages, plus Fall Styles: 32 pages
Residential Property: 80 pages
Long Island Weekly: 28 pages, plus Fall Styles: 48 pages
Westchester Weekly: 38 pages, plus Fall Styles: 24 pages
Connecticut Weekly: 40 pages, plus Fall Styles: 20 pages
It is worth noting that there were a ton of ads dominating the inside pages! That's how they could afford to print so much.
Other than the inclusion of the "Fall Styles" section in the regional sections, this was not too much of an outlier -- the next Sunday paper was archived at 912 pages, again including regional sections. The next year's "Fall Styles" issue (Sept. 11, 1988) was archived at 1010 pages.
By contrast, twenty years later (in September 2007) the New York Times Sunday edition was down to 672 pages as archived. So still gigantic and still a ton of ads, but about 40% smaller than 1987.