[djur]

Data compression is really good these days, and there are file formats and compression strategies that are optimized for gaming. The specific technique being used here is still used in 2D games, but there are spritesheet optimization tools available to do it automatically.

My understanding is that for the biggest games there's an intentional tradeoff to use more storage in exchange for faster load times, although the need for that has apparently lessened over time. If you go back and look at some of the big multi-disc games of the late '90s/early '00s you might be shocked by how much duplication there was in order to reduce the need for disc swapping. (You might also be shocked by how much of that disc space was required solely to support pre-rendered cutscenes.)