Use the COMPRESS command to reorganize the contents of  a  dictionary
  file.   The new dictionary file will probably be smaller than the old
  file, since only the directories and data definitions are copied, not
  the   unused   free  space.   Only  valid  dictionary  files  may  be
  compressed.  You must  use  the  FIX  command  to  repair  a  corrupt
  dictionary file before you can compress it.

  To compress the main dictionary file, you  need  VMS  SYSPRV  or  VMS
  BYPASS privilege.  To compress a subdictionary file, you need only be
  the owner of the file.

  Command Syntax:

  COMPRESS input-file-specification output-file-specification