Optimizing for disk space is very low on the priority list for pretty much every game, and this makes sense since its very low on the list of customer concerns relative to things like in-game performance, net code, tweaking game mechanics and balancing etc.
Apparently, in-game performance is not more important than pretty visuals. But that's based on hearsay / what I remember reading ages ago, I have no recent sources. The tl;dr was that apparently enough people are OK with a 30 fps game if the visuals are good.
I believe this led to a huge wave of 'laziness' in game development, where framerate wasn't too high up in the list of requirements. And it ended up in some games where neither graphics fidelity or frame rate was a priority (one of the recent Pokemon games... which is really disappointing for one of the biggest multimedia franchises of all time).