Oddly? The game needs ram to store data like variables that the game generates, like physics simulations, among other game systems. The game's asset size alone doesn't really matter.
Nah, most of the space is filled with textures in a graphical game. Which is odd in 2:1 RAM:disk ratio, since most of the textures are in ddx nowadays, a format the GPU can use 1:1. You can't really compress ddx.
Still has to be put somewhere, for speed of access. Directly disk to vram would prkbably feel like the chunk loading in Kenshi (engine is old and devs were amateurs).