Bill Leeney gave me the following info that should help explain what you are seeing:
Space is allocated to a collection/doctype when records are inserted.
This space remains allocated, even when records are deleted (exception for the special case below).
Deletion of records does not imply an increase in free (unallocated) space.
Special case ? if all records are deleted I think a refresh is done internally (at least in later versions of Tamino), and this causes all allocated space to be returned to free space pool.
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