[Zope] Size of a zope export

I experiencing some unexpected behaviour when performing an export. I'm trying to minimize the size of the export by removing a lot of data that isn't needed. I delete a few thousand objects and call manage_catalogClear(). However, since I still want to keep this data I put all this code in a python script and raise an exception directly after performing the export. This roles back the transaction so all the data is still retained. The problem is that the zexp file is the same size as if I hadn't deleted anything! Anyone know why this would be or another way to export a subsection of objects rather than the whole lot?