It's the other way around - because pieces cost roughly based on their size (amount of material) modern Lego sets are "denser" and heavier on average than similar sized sets of the past, because as piece count (and detail) goes up, piece size has been going down.

The discussion is about price, not cost. Lego is keeping the price per piece somewhat stable because they know people look at that, but as pieces get smaller (and thus cost less to make), their margins go up, and sets get smaller for the same price.

The set that started this subthread is very much an outlier in that regard. Usually it looks like this: https://www.reddit.com/r/legostarwars/comments/1lpuz4d/lego_...