FAQ-Wiki
Q:Why size of result pst file smaller than size of source .ost file in several times?
A:
The size of the converted .pst file shows the actual size of information stored in the ost file. If you do not make archives and do not start Cleanup Tool regularly for the original Offline Storage Folders, then the .ost file size is only growing. As a result, only 5-10% of the file can be used for a real user's data.