The current documentation on Archiving a project and Archiving an issue could be improved to note the Index size will only assertively decrease after a re-index in Jira.

The Archiving a project does mention the need of a re-index:

3. Re-index Jira to remove the project's data from the index. You can do it after archiving multiple projects or whenever you see fit. Re-indexing is needed to get the full performance benefits.

— but doesn't make it clear even the Index size (and disk space) will only assertively decrease after the re-index.

Related (and recently published) KB article:

• Jira index size and disk space are not decreasing after archiving Issues or Projects or cleaning up