The offline access guide that is posted on PowerLink explains how Offline Access works with the OST files. Basically,
The Offline Access Agent on the user’s computer synchronizesthe OA cache with the OST file.
shortcut messages are created in the user’s mailbox on the Exchange server and replicated to the OST file on the user’s
Cache settings can be modified and the details of how configure cache is documented in the offline accesss users guide, also on PowerLink.
We can use the PSTNullFreeOnClose registry key. However time it takes Outlook to shutdown and if Outlook is forced closed during the compaction process the data file could become corrupt. If you use both this key and the option to empty deleted items on exit, Outlook will take much longer to exit.
However, this setting can be useful to security conscious users as compacting removes all traces of deleted messages, making it impossible to recover the mail.
If you want to force Outlook to return this free space every time that you close Outlook, you can add PSTNullFreeOnClose to the registry. (Replace xx.0 in the path with your version of Outlook.)
Value Name: PSTNullFreeOnClose
Value Data: DWORD 1 = compact on close, 0 = don’t compact
Copy paste the text in notepad and save it as .reg and run to make required changes as per your outlook version.