A point worth noting, I think … Say your Mailbox as reported in Exchange Management Console/Shell/ESM is 110 Mb, an often asked question is why the OST file for that mailbox is soooo much bigger.
Microsoft have the answer : http://office.microsoft.com/en-us/ork2003/HA011402591033.aspx
When deploying Cached Exchange Mode for Outlook, be aware that users’ OST files can increase in size by 50 percent to 80 percent over the size of the mailbox reported in Exchange Server. The format Outlook uses to store data locally for Cached Exchange Mode is less efficient than the server data file format, resulting in more disk space used when mailboxes are downloaded to provide a local copy for Cached Exchange Mode.
And further.. Remember the OST/PST “speed up” patch which was released. It had a small side-effect :
Hope that’s interesting !