Outlook mailbox storage
Outlook can keep years of mail and attachments in one data file. Deleting a few large messages does not necessarily make the file smaller on disk right away.
Scan the Windows drive and look for Large Outlook data files in SizeTrend's Overview pane. It lists offline mailbox copies and personal Outlook data files by their measured size, including files outside the location you normally remember.
A user can have several profiles, archive files, old PSTs, and multiple OSTs. Finding the filename and path is the first step. It is not yet permission to delete it.
An OST is usually an offline copy of a mailbox stored on Exchange or Microsoft 365. Outlook can often rebuild it from the server, but deleting it while Outlook is running or before confirming the mailbox state can cause a long resynchronization and may complicate troubleshooting.
A PST can contain the only copy of messages, calendar items, contacts, and attachments. Old does not mean redundant. Back up and open the PST in Outlook before deciding that it is no longer needed.
Outlook data files reuse internal free space. When messages are deleted, the PST or OST may keep the same Windows file size so Outlook can reuse that space later. Empty Deleted Items and allow Outlook to finish its work before checking again.
For a PST, Outlook's Compact Now command can return unused internal space to Windows. Compaction can take time and should not be interrupted. An OST is normally better controlled through cached-mail settings and mailbox synchronization rather than manual file operations.
SizeTrend's Manage action opens Windows Mail or Outlook profile settings when available, and otherwise selects the data file in Explorer. In Outlook, use Mailbox Cleanup to find large or old messages, review attachment-heavy folders, adjust how much Exchange mail is kept offline, or archive material according to your organization's policy.
Do not compress, move, or delete an open PST or OST in Explorer. If you are retiring an old PST, close it in Outlook first, confirm that it is backed up, and make sure it does not contain the only copy of anything you need.
If the data file keeps expanding, save a SizeTrend scan and compare it later. That will tell you whether Outlook is responsible for the continuing drive growth or whether another application is filling the same disk at the same time. Saved-scan comparison is available during the 14-day Professional evaluation and with a Professional license.
Mailbox quotas and server retention do not necessarily limit the size of every local archive PST. Old archives need their own review and backup policy.
Find the data file, then manage it through Outlook.
SizeTrend shows the large PST and OST files without treating either one as disposable cache.