We have already seen how we can use the built-in Reg.exe command-line utility to perform almost all the task which you otherwise can with regedit.exe. You can also use Reg.exe to save a registry key to a hive file.
But if you find that the Reg.exe utility does not compress a registry key when the utility saves a registry key to a hive file, you can apply this hotfix released by Microsoft.
When you try to make copies of the registry keys, using the reg save command, when you export the registry key into a hive file, you find that the registry key does not compress the key during the exporting process. As a result, the size of the exported key is very large. In such a scenario you may apply this hotfix from Microsoft.
This hotfix will correct the issue, which you may be facing on a system running Windows Vista, Windows Server 2008, Windows 7, Windows Server 2008 R2, Windows 8 or Windows Server 2012.
After you download and install this hotfix 973817, you can use the /c switch in the reg save command to compress the registry key.
For instance, this command will compress the HKLM\System subkey:
reg save hklm\system %windir%\system32\config\system.lkg /y /c
Hope it helps solve your problem.