[Zope] Zope database file grows when no one is modifying it

I am administering a Zope/CMF site running Zope 2.6.4 with python 2.2.3. We have been running low on space and have not been able to clean out the database of old edits. We are moving other sites over to a new server and were able to free up about 3GB. Unfortunately, our main Zope site has filled up most of that space quite rapidly. We haven't edited it much so it is unclear why it is growing so quickly. What might be amiss? Thanks! Jamie