We have a Windows project with an on-disk size of “11.8 GB (12,692,746,240 bytes)” and a Library folder with an on-disk size of “9.99 GB (10,731,413,504 bytes)”.
The Library folder is a folder that is automatically created by unity without any additions on our part, but its use is unknown, yet it occupies more than 90% of the project size, which is very inconvenient.
How can I reduce the size of the Library folder?
The library folder is unitys internal representation of everything in the project. The folder can be removed but will be automatically regenerated when reopening unity.
The only way I know of to reduce the size of it would be to remove or compress any unused assets in your project. You can also first try deleting the library folder and letting it regenerate, it may be smaller than before if you do so.
Just be sure to keep the Assets, Packages, and ProjectSettings folders intact.
Hope this helps!