Windows 10 Windows.old Size Shows Differently Across Tools

Run Disk Cleanup as an administrator, check the box for Previous Windows installations, and scan again.

The discrepancy in the results is due to differences in how various tools track hard links and compressed files.

The huge discrepancy is because each of these tools is displaying a different aspect of disk usage. Windows.old is a separate folder that contains a copy of your previous Windows installation, while Disk Cleanup and File Explorer are showing the current disk usage.

Sometimes, Disk Cleanup's estimates are more conservative or less accurate, especially if files are marked to be deleted but haven't been physically removed yet, or if it accounts for compression and system file considerations.

WinDirStat and Disk Cleanup use various methods to scan the file system, which can lead to differences in reported sizes. WinDirStat scans the file system recursively, while Disk Cleanup uses a different approach that might miss some files or folders.