Game installations have grown far faster than the size of the worlds they contain. The reasons are mostly technical decisions about loading speed and distribution rather than more content.
Compression is traded against loading time
Assets can be stored compressed, which saves space but requires the processor to decompress them before use. That work happens while the player waits.
Storing assets closer to their final form removes the decompression step, so levels load faster. Studios facing complaints about loading times take the space.
Some engines go further and store duplicate copies of assets arranged so that everything needed for one area sits together on disk. Duplication trades capacity for fewer reads.
Audio is larger than most players assume
Voice acting in several languages multiplies the same scene many times over. A story-heavy game with full localisation carries most of its bulk in speech.
Heavily compressed audio saves space but introduces artefacts, and dialogue is the content players notice artefacts in most readily.
This is why selective installation of language packs became common. Removing unused voice tracks often frees more than removing anything else.
Textures scale with resolution in two dimensions
Doubling a texture's resolution quadruples its data, because the increase applies to width and height together. Higher-resolution art grows sharply rather than steadily.
Games shipping high-resolution texture options therefore carry a large optional payload, often larger than the base game.
The visible improvement is modest at normal viewing distances, which is why these packs are usually separated. The size is real even where the benefit is marginal.
Patches add rather than replace
Update systems often download new files and leave the old ones in place until a cleanup pass runs. Installations therefore drift upward over a game's life.
Games built from large packed archives face a worse version of this, since changing one asset can require rewriting a large container file.
This is why reinstalling a long-running game sometimes recovers a substantial amount of space with no change in content.
Storage economics removed the pressure
When capacity was expensive, size was a design constraint and teams spent effort minimising it. That effort has an engineering cost of its own.
As storage became cheap the incentive weakened, and the work moved to problems players notice more, such as loading and stutter.
The constraint has partly returned through console storage limits and download caps, which is why selective installation and asset streaming are receiving attention again.