I realize this post is a bit old, but I wanted to thank everyone for their suggestions and give our experience to back up what has been said in this topic already. We experienced an image that should have been about 120gb, but was showing in FOG as the full 500GB hard drive. After referencing this thread, it fixed the issue. The steps we took are below in order:

Verified that the PC didn’t have a second drive or second large partition it was capturing
Verified that bitlocker encryption was OFF using the command prompt command “manage-bde -off C:”
Ran Disk Cleanup (to get rid of old unused files)
Ran Tools > Check to scan for hard drive errors (may require reboot to fix)
Ran Tools > Tools > Optimize and defagment
Capture Image again

Once we did those steps, the newly captured image only takes up the actual amount used on the hard drive.