If a particular title is giving you trouble you can export to a PKG file then import it with a different name. That ensures disk usage calculations are not including resources that were used in previous revisions of a title. Resources are kept if they were used in any previous kept revision of the title. Reducing the number of revisions should help with this, but exporting and importing will give you a clean reading.
Click the “Disk Usage” link on the Lectora Online Getting Started page after logging in. This should give you a detailed report for your titles.