The first section is very highly detailed with lots of unique assets with 4k textures. I am aiming for realism with AAA quality graphics. This first section contributes to a considerable chunk of the file size, and although the full game will be much bigger in scale, it won't be that much bigger in file size.
I'm also just a solo dev. I've done my best to optimise the game for performance, but I don't quite have the luxuries of a team to specialise in optimisations in general