Why Are Game Installation Sizes Still Increasing?

In 1989, the first Sierra game I ever played, Space Quest III, shipped on six 5.25-inch double-density, double-sided floppy disks. A hard drive was optional, though copying the game files to the HDD substantially improved performance. The total game install was between 3.5 and 4MB–but that was significant, when a computer from the same time period only had between 10 or 20MB of HDD space.
Fast forward nearly 30 years, and games of today commonly eat dozens of gigabytes per install, dwarfing the sizes that were common as recently as five years ago [...]