Ultima Online is a 25 year old MMO that still has the original servers running. The install is about 3GB and can run on any PC from the last 20 years. For the official servers, the player base is largest on Atlantic and you can sign up for a free Endless Journey account. PVP is only allowed in certain zones.

There are many many player run servers that fit a lot of play styles. The most populated being Outlands. That is where I mainly play as it is by far the most populated UO server, 2.5k-3k people on usually. Just beware, outside of the starting zone and towns, it is open PVP almost everywhere.

City of Heroes now has player run servers. The install is around 5GB. Homecoming is the most populated, with a lot of added content. Rebirth is less populated but tries to be the closest experience to the official servers.

Lord of the Rings Online is about 26Gb.

Star Trek Online is also roughly at the same ballpark as LOTRO.

Guild Wars 1 is about 5Gb.

Secret World Legends also this one, about 10Gb.

They are all decent, and fun to play if they’re your jam, some are more pay-to-win than others, like Star Trek Online. Some are a bit on the older side, like Guild Wars 1 being from 2005 though.

Project Gorgon 20GB. Very old school, skill based, slow paced, made by 2 indie devs that worked on things like Asheron's Call, Star Trek Online, etc.

Dungeons and Dragons Online is 32gb.

Here's one that most of you have probably not heard of. Monster Hunter Frontier.

MHF was a Monster Hunter MMO that ran from 2007 to 2019 and was exclusive to Asia. Recently, a dedicated team of community members have managed to revive it with community servers.

It is brutally difficult, only partially translated, and has some genuinely awful controls, BUT

It's only around 6GB and completely free.

If for some reason you want to try it I highly recommend joining the rain server, as it is the most populated and stable one. They also have a setup guide for how to install on their discord server I recommend you use.

RuneScape (both old school and modern version) are MMOs that use very little space, at most only a gig or two “cache” is downloaded.

What about MUDs? I haven’t played them myself but would love to try one afternoon.

Realm of the Mad God is a fun little roguelike MMO.

Well, there's Tibia, and there's MapleStory

I'm currently playing on one of many wow private servers. Wotlk client, blizzlike 1x experience, and it's only 7GB

Ragnarok online on a private server if you don't mind a bit of clunk. Very bright and colourful anime art and kickass soundtrack.

i'm definitively done with WoW, but i remember having a good time with the classless private server Ascension. The gimmick is that you put together a build of not just talents, but spells too, so you could be an archer that uses frost effects instead of snares and a pet, or a sword and shield combatant that uses shadow magic. I think they're on the TBC expansion, but using a Wotlk client atm, which means it takes up very little harddrive space.

I believe you are actually looking for a SmolMORPG

Project gorgon might be worth a look, its pretty good relatively small mmorpg

Always project1999 with its decent sized community if you want a very small retro mmorpg.

The Mama World (Manaverse) is pretty small IIRC. But it's a sprite style game, not sure if you're after that.

Haven and Hearth is a slow base building MMO. You build up skill points exploring and finding new items, spend the skill points to unlock more stuff, build a base and start upgrading your stuff from there. It's honestly really cool.

Puzzle Pirates!

It might not be what you're looking for, but Temtem is listed as like 5GB on Steam. It's basically Pokémon except built as an MMO. The entire main story can be played in co-op, but I haven't found any of the dungeons or whatever yet as my friend and I have been doing a lot of exploration and side quests.

If that sounds interesting at all, it's part of this month's Humble Choice for $12 and that's the best deal I've seen on it. Just remember to cancel your subscription after redeeming the games in the bundle.

There are some older Neverwinter Nights and Neverwinter Nights 2 persistent worlds still operating. The graphics on both games are a little dated at this point, but if you're looking for small player population and good storytelling, this may just be the ticket.