The memory used for the Registry is approximately equal to the size of a hive when it is loaded into memory. Hives vary in size on disk from 20K to more than 500K. The amount of space used depends chiefly on how many local user profiles are retained and how much information is stored in each profile.
You should remove unused or out-of-date user profiles from a computer by using the Delete User Profiles command in Windows NT Setup. (The Setup program protects you from deleting the profile for the currently logged on user.)
You can use the Save Key command to save a user hive, and then use the Restore command so you can use this smaller hive. How much space you gain depends on how much was stored in various user profiles.
This procedure is useful only for user profiles, not for the SAM, Security, Software, or System hives.