When you are in the Zope Management Interface (ZMI) and looking at the Database tab, you see both the location of the ZODB (Data.fs) and the size. Remember that the size of the ZODB is not just the items (objects) that are in it but all revisions since the last packing. Meaning, for every document, folder, object change, Zope will keep a copy of that change in your ZODB. Packing, eliminates those changes from your ZODB.

The Cache Parameters tab gives you the number of all objects in the ZOBD and the number in memory (cache) which is a sum of all the different Zope connection threads and the amount of objects each one has in cache.

To change the total Zope memory usage, lower the objects per cache number and click Change.