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.