iCloud on a PC downloads your photo library unless you stop it

iCloud for Windows keeps iCloud Drive and, if you let it, your entire photo library on the PC. The photo library is the part that surprises people, because on an iPhone it is optimised and on a PC it is not by default.

Where it stores

Get-ChildItem "$env:USERPROFILE\iCloudDrive","$env:USERPROFILE\Pictures\iCloud Photos" -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); Path = $_.FullName }
  }
Get-ChildItem "$env:LOCALAPPDATA\Apple Inc" -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); Path = $_.FullName }
  }

The settings that matter

1. Open iCloud for Windows.
2. For Photos, choose Optimise rather than downloading originals, or turn the download off entirely.
3. For iCloud Drive, files support on demand in the same way as OneDrive: right click a folder and free up space.
4. Check the Photos download location, which defaults to Pictures and can be moved.

The difference from OneDrive

Both use Windows' cloud file placeholders, so both behave the same in Explorer. The difference is the default: OneDrive ships with files on demand on, and iCloud's photo downloading is a choice made during setup that most people click through without reading.

Two clients on one PC

A machine with both OneDrive and iCloud is keeping two sets of local copies for two services, and a photo saved to both exists twice. Deciding which service owns photos, once, is worth more than any cleanup.

The equivalent question on the other platform, with the settings for each service, is in what sync folders keep locally, and OneDrive on Windows covers the placeholder mechanics in detail.