This article explains how to reduce disk space for a wiki. It will focus on English Wikipedia, but the same steps apply to any other wiki.
The main BZ2 file takes 9 GB. This BZ2 file is located wherever you downloaded it.
This file is not used by XOWA, and can be removed from the system.
If you are using the automated set up from Dashboard/Import/Online, the BZ2 file is moved to /xowa/wiki/#done. XOWA does not delete this file in case the user wants to re-import the wiki, and avoid redownloading the entire file.
You can prune entire namespaces. For example, some users may not be interested in the Wikipedia namespace as it contains mostly meta-information about Wikipedia. The namespace itself is about 3.0 GB.
To remove this namespace, you can delete the entire namespace db. The file would be en.wikipedia.org-text-ns.004.xowa