I'm in need of playing something with epic story, so RPGs with big story quests are what I'm after. Preferably published past the year 2018, cause I played many games before that date and I feel I need something more modern.

It also needs to be fast paced (as in story progression, not combat) and small in size (regarding gigabytes). Something like Ghost of a Tale or Thea 2 should work, since their story is fast paced and they don't have big installation size.

Hm, that's a tough one. Epic RPGs are usually filled with pace retarding side quests, trash mobs and padding, while the games with fast-paced stories tend to be shorter and with less choice. And the year adds an additional hurdle ...

I assume you have played Shadowrun: Dragonfall (2014, 2GB)? Or Tyranny (2016, 15GB), which is said to be a bit shorter and focused on story, if I'm not mistaken?

Is Thea a story-driven RPG? Always thought it was more like a strategy game, but I haven't played it.

When I look at 2018+ games I have played, I see no actual RPG on there with a fast paced story, I can only come up with Unavowed (story-driven point-and-click adventure, 3GB) and Hedon (first person fantasy shooter with prominent story and exploration elements, 0.5GB) ...

Or maybe the Tyrants of the Moonsea DLC adventure (2019, 10GB) for Neverwinter Nights EE, in case you have the main game?

I think Baldur's Gate 3 fits your description (except for the small in size part, I guess), at least from some of the playthroughs I've seen... too bad it's not yet finished. Yet to the "relatively insignificant, powerless protagonist", I'm sure the whole adventure was more than epic :P.

I'd say Tower of Time is a pretty epic story, it's around 6GB.
Not sure what you mean, fast paced story but not combat. Most epic stories are a bit slow that I can think of.
My playthrough was about 45 hours. It has tons of lore and reading if you choose to.

Might not fit the criteria except for the "epic" one but I thought it can't hurt to mention it anyway:
Kingdoms of Amalur: Reckoning