Hello John,
Any difference in disk space usage between Hauptwerk v4 and v5 (in any given situation) should be negligible. I don't know off-hand whether a clean macOS 10.13 installation needs more disk space than a clean installation of any given pre-10.13 version, but in general newer versions of operating systems do often need some more disk space, and also upgrading an existing operating system installation may well end using more disk space than performing a clean installation of the new operating system version.
When you upgraded from Hauptwerk v4 to v5, did you follow these instructions to delete the folders mentioned in step 3 completely before installing v5?:
viewtopic.php?f=10&t=17850N.B. in particular, since v5 installs to a different location from v4, if you hadn't completely deleted the last three of those folders, i.e. this one:
3. Use Finder to make sure that all of the following folders are completely deleted (if they exist). You may be prompted for your password:
...
'/Hauptwerk'[/list]
[The latter folder is at the very top-level of your computer's SSD/hard-drive. In order to be able to find it, make sure that the 'Sidebar | Devices | Hard disks' option is ticked in Finder's preferences.]
... then your old v4 organ files/data would still be in that folder occupying disk space.
Best regards, Martin.
Hauptwerk software designer/developer, Milan Digital Audio.