So I've done some digging, and the size seems to be slightly spread out into different folders, all of them with a noticeable difference between size (S) and size on disk (SD);
- src S:27.7MB | SD:2.23GB
- res S:113MB | SD:1.80GB
- bin S:46.1MB | SD:12.7GB
within bin\com\lilithsthrone\ , two big folders stand out:
- game S:38.8MB | SD:10.4GB
- dialogue S:16.3MB | SD:5.55GB
There's normally a bit of a disparity between the actual size and the "size on disk" fields, but that's pretty extreme.
Basically, the reason there's a disparity at all is because file systems are divided into allocation units, or block sizes. Usually this sits around 4KiB (on modern systems like NTFS, APFS, and ext4, at least), so every file on the disk, regardless of how small, will consume at least that much space. My guess is that you just have a metric ton of tiny files floating around in your local repo.
If your repo lives on a largeish FAT32 volume (some external drives still ship formatted as FAT32 for maximum OS compatibility), that would also cause a greatly inflated size on disk figure (
edit: the reason being that on larger drives the block size for FAT32 is a whopping 32KiB).
It's not exactly an apples to apples comparison, but on my Mac (on an APFS volume with a 4KiB block size), my LT repo comes out to around 530.8MB (580.7MB on disk) with 20,848 files in the directory. I'm not at my Windows box right now, but I don't think it's too far off from that (
edit²: Just checked my Windows rig. S: 512MB, SD: 551MB)
If you're desperate for a quick fix and/or your disk space is at a premium, right-click the Lilith's Throne directory, click "Properties", click the "Advanced" button, and select the "compress" option. Hit OK a bunch of times, wait for it to think for a while, and the size on disk will likely dip below the actual size, at the expense of some performance (probably won't even be noticeable in practice).