Unity keeps an editor per version and a Library per project

Unity re-encodes every asset you import into a form the target platform can use, and caches that per project. The rule that follows is the useful one: the folder you should not back up is usually the folder that is largest.

What is in a project

Get-ChildItem $env:USERPROFILE -Recurse -Directory -Filter Library -Depth 4 -EA SilentlyContinue |
  Where-Object { Test-Path (Join-Path $_.FullName '..\Assets') } |
  ForEach-Object {
    $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
    [PSCustomObject]@{ GB = [math]::Round($s/1GB,2); Path = $_.FullName }
  } | Sort-Object GB -Descending | Select-Object -First 15

The check for an Assets folder alongside is what stops that matching every other folder called Library on a Windows machine, of which there are many.

Editors accumulate

Get-ChildItem 'C:\Program Files\Unity\Hub\Editor' -Directory -EA SilentlyContinue | ForEach-Object {
  $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
  [PSCustomObject]@{ GB = [math]::Round($s/1GB,2); Version = $_.Name }
} | Sort-Object GB -Descending

Each editor is several gigabytes plus a module per build target you added. Projects pin a version, so old editors accumulate. Remove them through Unity Hub rather than deleting folders, and check each project's ProjectSettings\ProjectVersion.txt first.

The shared caches

The Asset Store download cache and the global package cache live under %APPDATA%\Unity and %LOCALAPPDATA%\Unity, and hold every package you have ever downloaded. Both are redownloadable and neither is cleared by anything.

Platform switching doubles the cache

Switching build target reimports assets for the new platform and the cache keeps both, which is usually why a Library folder is several times the size of the assets. The same shape on the other platform is in Unity projects on a Mac.