In Windows Server 2016, a huge difference between "Size" and "Size on Disk" typically indicates one or more of the following factors:
1. Sparse Files

Some files are "sparse," meaning they appear larger than they actually occupy on disk. This happens if an application creates large files but only writes data in some portions, leaving gaps that don't take up physical disk space.

2. Compression (NTFS Compression or Deduplication)

If NTFS compression or data deduplication is enabled, files take up less space on disk than their original size.

3. Cluster Size Mismatch

The disk is formatted with a large allocation unit size (e.g., 64KB clusters), meaning small files waste significant space. A large allocation unit size is common in some file systems optimized for large files.

4. Hard Links

Files that are referenced in multiple locations (e.g., symbolic links, hard links) do not take up additional disk space, reducing "Size on Disk."

5. Alternate Data Streams (ADS)

Files can contain hidden alternate data streams that increase their reported "Size" without affecting "Size on Disk."

6. Virtualized Storage (Thin Provisioning)

If the file is stored on a thin-provisioned virtual disk, Windows may report a larger logical size while using less actual space.

7. Reparse Points and Deduplicated Files

Files using NTFS reparse points (e.g., deduplication, OneDrive files, or symbolic links) can appear large but occupy little space.


How to Investigate Further

Check if NTFS compression is enabled:

Right-click the file/folder → Properties → General tab → Click "Advanced."
Look for "Compress contents to save disk space."


Check for Sparse Files:

Open Command Prompt as Administrator and run:fsutil sparse queryflag "C:\path\to\file"

If the file is sparse, it reports: This file is set as sparse.


Check Deduplication Status:

If deduplication is enabled, files may appear much larger than the actual disk usage. Run:Get-DedupStatus




Check Cluster Size of the Disk:

Open Command Prompt as Administrator and run:fsutil fsinfo ntfsinfo C:


Look at Bytes Per Cluster (e.g., 64KB clusters will waste space on small files).


Check for Hard Links:

Run:fsutil hardlink list "C:\path\to\file"


If multiple locations are listed, the file is shared between them.




Would you like help checking any of these on your system?