When you’ve got a large, diverse, growing record collection you notice that they are not all the same. I see that since the 1960s the amount of vinyl used has been systematically decreasing, and it also varies with label and country of origin (i.e. which pressing plant made it). My early 1970s LPs are typically 155g, and by 1979 the typical weight was 110g. In the early 1980s Warner Brothers (for Beggars Banquet in the UK – a very apt name, indeed!) were producing records just 80-100g in weight. Where was the heavy rock? It all started in 1969 with the dreaded flexible (flimsy) Dynaflex from RCA! More on that here and at Stereophile here. Although derided as “dynawarp” by some, and intended to present reasonable sound quality on low-performance players by using a compression and EQ algorithm, the Dynaflex discs were made with 100% virgin vinyl rather than the ground recycled material commonly used by other manufacturers.
There’s a historical account here.