I have users that use Outlook as their "file cabinet".. Theyll create huge numbers of new folders under "Personal Folders" and drag drop items from Inbox to these folders in Outlook. These extra folders (other than the normal folders supplied by Outlook) do comprise parts of the .pst file and add to it's size and accompanying bloat. I can harp all day about Inbox/folders - I can't get them to drag/drop the items to a Windows Explorer structure, removing them from the effects of .pst bloat.
From my standpoint, Inbox is not the only factor contributing. Still, this was a valuable article for me who has no choice but to use Outlook.
Keep Up with TechRepublic