The space could probably be optimized better, but I'm not really sure how - the technical details of optimization are definitely my strong suit. Most sprites are '#' x 1500 though all the character sprites are individual files than some kind of conglomeration. There's about 1GB of sprites, then around 650MB of sound (which will increase once the rest of the game's audio is in). The sprites for the particle systems/attack effects in battle are pretty sizable, past the 2GB point. I could probably reduce the file size if I changed them from PSDs to PNGs. Probably worth experimenting with once I'm done with the game's content. Since the battle effects are something I purchased from the Unity Asset Store and not something I made myself, I was hesitant to change anything and mess things up.
OK, what do you mean by “’#’ x 1500 though”? 1500x1500? To support 1440p screens?
1GB of sprite doesn’t look that much… or you’re talking just about characters, and FX and backgrounds add much more? (I felt like characters were the biggest, but it’s true that FX frames can quickly add up).
On my Renpy project I encore most sounds to .opus and pictures to .webp max quality for high quality and low space usage. However, Renpy uses assets directly. Unity compresses them with its own algo, so I’m not sure it will make a difference in build. Import settings, esp. format, and texture packing settings probably make the biggest difference. But I’ve never worked on a game with so many assets (except in an indie studio that was making a HD 2D game with many backgrounds and detailed props at 4K res - probably the 2D game that took the most space I have ever installed on Steam!).
It is possible that Unity tries too hard to keep layers on the PSD, though. I would imagine that it renders to a flat picture on build, but I’m not sure. You can try and export some of the biggest PSD to a flat format (png or webp) and then see how it affects build size.
For the battle FX I can’t tell, but you can evaluate their size in build by building a sample project just with them. Same remark for your PSD / flat PNG. Assuming they are packed on different spritesheets so there is no interaction between them.