I love playing Dwar Fortress, I've spent hours and hours in there.
The game is free from the developera site, the download is only 15MB.
If you want to support them you can buy it in steam, the listed requirements is 500MB of storage, I assume since this version has a tile set.

I've also put so far 400 hours in oxygen not included, I think it uses around 2GB of storage.

And to me, any monster hunter game its worth its price, I've bought each game and played it for minimum 200 hours each, I think I reached 500 in on of them.
Tho the newer ones are pretty heavy for their respective platforms. Also triple-A game price.

DuckTales erotic fanfiction and Z-code files.

Weighing in at 4.0MB, I present to you the SNES roguelike Mystery Dungeon 2: Shiren the Wanderer (or Fushigi no Dungeon 2: Fuurai no Shiren).

The original console "RPG you can play 1000 times". It's tough but fair. It stops just short of permadeath; dying sends you back to the start at level 1 with nothing, but you keep your side quest progress and any gear you had the foresight to send back to town before you died. Watch someone stream this sometime. It's turn-based, but the tension is like nothing else I've ever played.

Soem years ago I played a few hundred hours of Terraria and was always surprised how much enjoyment you could get out of the ~ 30 MB that it was when installed. Don't know about it today though since it has received quite a few updates since then.

Alright then, how about this?

For just 32kB (plus the size of a Game Boy emulator), you can play the amazing Tetris Rosy Retrospection. It's a romhack of the Game Boy Tetris that adds modern Tetris controls, handling, and features to make it feel much better to play without increasing the file size. I'm aware of the color version of this hack, but it doubles the file size to 64kB, so I'm only considering the regular Tetris Rosy Retrospection this time. Byte-for-byte, I can't find a better game to sink dozens of hours into.

Nethack is under 5MB, runs on 80s hardware and you can play it forever.

Slice & Dice is 60-100MB (depending on what version you get), and I've been playing it for hours a week for 3 years now.

I linked the itch.io page, but It's also on play store/app store/steam for cheaper.

Animal Well came out this year and is 33MB. Great game, highly recommend.

for music you can get into .MOD files, theres millions online and you can edit them and make your own also. you can also emulate computers like C64 and Amiga for endless fun.

I don’t think it’s got quite as much bang for buck as an ebook or Tetris, but .kkrieger is a first-person shooter with multiple weapons, multiple enemies, multiple levels, sound effects, and music, and only takes up 97,280 bytes of disc space.

Pretty remarkable piece of software engineering.

I know it's not the subject of this question but... Why is storage space size your metric?

Do you take pleasure in knowing your good experience came in a small package? Or are you storage space starved for some reason and that's what guides your quest?

SD porn.

GNU Emacs

The most enjoyable passions IMHO is when you unleash your creativity. Write with emacs. Compose music with musescore. Draw with krita. Or, better yet, do it off the screen.

.mod files for music. 3-5mins+ of music a piece with a storage footprint in the KB range. Mostly free, tons available online. Check out Mod archive for an easy web interface or ZXTune if you're on android.

There's an old game called "total annihilation", amazing game, I've played for probably a couple hundred hours over the years.It's file size is pretty small too though I can't remember off the top of my head.

It's an RTS, there is a more modern version (free I believe) with better QOL and graphics called "Beyond All Reason" though it's much larger