Outer Wilds is 8GB, and the best puzzle/mystery game I have ever played. The less you know about the game up front, the better.

Balatro is tiny, like 150 MB tiny. And endlessly playable. If you like poker and/or rogue likes it's pound for pound one of the best deals out there both money and memory wise

Balatro and Slay The Spire 2 are roguelikes that have a lot of replayability.

Trackmania, you can give it a shot with Nations Forever it's free on steam. Turbo/TM2 are graphically improved versions of it, 2020 is the newest one that most people play but has a lot of its features locked behind a yearly subscription. Go with the previous ones if offline play is a must.

Baba Is You is a real brain burner puzzle game that makes you think out of the box a lot of time.

If you havent yet played Portal / Portal 2, together they are about 20 Gigs, and are both very good, genre and generation defining games.

Animal Well 35Mb

Genre/type of game you like most?

Have you tried Inscryption

My indie action platformer No More Mages is in early access (with a free demo) if you feel like taking a shot at a truly unknown game! More being added soon.

Celeste, tricky towers

Both Hades and Hades II are about 11GB. Either one will fit or you could increase your limit just a bit and fit both.

I haven't tried it yet, but Mina the Hollower just released and seems really cool. Should be way less than 20 GB.

I developed my taste in PC games before I had a graphics card, so...

• Baba Is You, a minimalist puzzle game with a Portal's worth of perception bending
• Hypnospace Outlaw, a story-driven 90's-styled computer/internet simulator with a shocking amount of good diegetic indie music. Like, more than one entire album.
• Celeste

I've already played these games to death and back so I haven't really been playing them in the present tense

Factorio if you like building, optimizing and automating. Game is about 5 GB.

I enjoy unique story driven games. Two favourites are:

• Papers, please. <100MB
• Do not feed the monkeys. <1GB

Honorary mention to Not tonight. It's very papers-inspired, but unique enough.

Minecraft (Java Edition, the standard).

About 0,3 GB, and needs no introduction. Endless box of fun in a tiny cross-platform package.

I like the Ace Attorney trilogies; I'm playing through the Apollo Justice trilogy right now and thats about 19gb. Perfect on the steam deck.

Stardew Valley is under 1gb and is also perfect on Steam Deck. Or Core Keeper is only 1.1gb, for a more crafty less RPG vibe.

Also worth remembering of course, you can use SD cards to expand the space on your Steam Deck, and swap SD cards out as much as you want. So if you have intermittent internet as you imply elsewhere here, loading up games onto SD cards can be very helpful.

Gotta plug my friends games, not that they're unknown, just the first things that come to mind and they're all small to small-ish in size (I think they might even be under 20gb for all combined).

Hotline Miami
Hotline Miami 2: Wrong Number
Raft
Sea Salt
Post Void
Keep Driving

Deep rock galactic

I'm going to go in a different, but important direction than everyone else: don't.

SSD's are constantly moving bits of data around to balance the amount of read/write each sector deals with, since that is what degrades the components.

The downside is that the swapping also counts as read/write cycles. Most of the time this is fine, because the SSD is doing relatively little data swapping compared to what it's optimizing.

This goes away when your drive is more than 90% full. Then your drive become one of those missing tile puzzles, and it starts shuffling lots of data around constantly to get things in their optimal place. This leads to the swaps drastically increasing the read/write cycles and killing the drive early.

If you really need to have a bunch of games saved on your Steam Deck, you're better off getting an SD card and using that for additional storage. While it's slower to load, it's not that much slower, and it's not noticable for smaller games

Have you considered something like EmuDeck? 25GB is dozens if not thousands of games from older retro consoles, depending on how old you would consider.

Dwarf Fortress

Throw money at the steam version or just got download the lazy noob pack (keyboard recomended for that one) and learn why it is the hardest game.

Remember kids, Losing is fun!

https://www.gog.com/en/game/a_hat_in_time

A Hat in Time, its a fairly small ~10GB game that plays like Mario 64 and its so gosh darn cute. <3

I've been enjoying Age of Empires 1 lately. Or Slay if you like puzzle games. Simon Tatham's Portable Puzzle Collection is also great for that. I dunno, there's so many options. Wanna be anymore specific about the type of game you're looking for? It seems a bit silly to mention nearly every game ever made

Lot of my picks have already been mentioned but if you like story-rich point and clicks:
The rusty lake series (~1GB each)
Strange Horticulture (1GB)

Or very chill adventure:
Slime rancher (1GB)

Rouge-lite/rogue-like:
Death Road to Canada (60MB)
Binding of Isaac (449 MB) - this is my all time favourite game and I'm surprised to see the base game is still under $20.

Fields of Mistria is a cute farming game like Stardew Valley, and it’s only like 800MB.

Horsey Game. Like 150mbs and hilarious but also crazy deep genetics mechanics.

Megabonk is like half a gig, great on Deck, pretty addictive, and good for when you only have a short window to play something in.

I have been playing Time Snatcher Handy recently, a roguelike centered around a time-stop mechanic, which only uses 44 MB. It is pretty good. Going through my steam library from the lowest up, here are the others i would recommend, stopping at one GB, because most of my games are under 20 GB and i tend to be relatively selective with what games i get so i consider most of them good also:

• TIS-100 (80 MB), programming puzzle game by Zachtronics
• Islanders (150 MB), minimalist city builder
• Enter the Gungeon (346 MB), bullet-hell roguelike
• Streets of Rogue (417 MB), roguelike which emphasizes having multiple possible approaches for how to get any given objective
• Opus Magnum (493 MB), another Zachtronics programming puzzle game
• Vault of the Void (554 MB), roguelike deckbuilder (one of my most played games, especially recommended)
• Backpack Hero (616 MB), roguelike focused on maximizing adjacency bonuses on your inventory grid
• Urbek City Builder (685 MB), city builder (obviously)
• Slipways (735 MB), a game about optimizing trade routes between planets
• Cassette Beasts (1 GB), similar to Pokemon, with the main differences being that you turn into the monsters instead of summoning them, type advantage/disadvantage giving unique status effects rather than just changing damage and battles generally being 2v2 instead of 1v1

There are games over 20GB?!

Celeste, HK, HK:silksong, hyperlightdrifter, titan souls, crypt of the necrodancer, journey, gris

You can even download 3-4 games from this they are rly smol

Brotato!

cult of the lamb and dave the diver are two of my go tos. would recommend!

one note tho: if you notice your deck acting slower, consider unintstalling some games. SSD performance tanks as it nears capacity. it's best to leave about 20% free. I personally don't always follow that, but when it starts acting slow thats my first bet.

Far cry primal.

Interestingly we are at this point that memory/chip prices are so high that we are asking for game suggestions based on storage requirements.

Undertale, In Stars and Time, Blue Prince, and Doki Doki Literature Club are my favorite games that haven't already been mentioned here. All of them are somewhat dialogue/text-heavy.