Originally posted by Jaggid Edje:

Cut scenes, high res textures, and audio tracks are the main things that take disk space for games. Valheim doesn't have any of those except audio tracks, and it has very few of those. So it's no surprise.

Audio tracks actually don't take much space, unless the game is very dialog heavy, with variations and etc. Textures is what takes the biggest chunk of the game's size. Keep in mind, that many textures have bumpmapping, specular, diffuse, alpha and displacement/parallax effects. Some even have surface subscattering textures and etc. So just one surface is actually 3, 5 or even 9 different images stuck together.
And then there are particles that are made of at least 3 textures - illumination texture, alpha texture and diffuse texture. Any game will easily gain up to 3, 10 or even 20 times its size if you apply a high-res texture patch to it.