The file will eventually automatically get reduced in sized ("compact'ed") by Outlook if you leave the PC idle with Outlook open, and the PST loaded.
You can also do it manually though. I've slightly edited the following guide from MS. You can checkout the whole guide here
Manually reduce the size of an Outlook Data File (.pst)
If you delete items from an Outlook Data File (.pst), the file is automatically compacted in the background when you’re not using your computer and Outlook is running. You can also manually start the compact command immediately.
Delete any items that you do not want to keep.
Open the Folder List view. At the bottom of the Navigation Pane, click the Folder List icon or press CTRL+6.
In the Navigation Pane, right-click the Deleted Items folder, and then click Empty Folder.
Click the File tab.
Click Account Settings, and then click Account Settings.
On the Data Files tab, click the data file that you want to compact, and then click Settings.
Click Compact Now.
While the above should work in most cases, it didn't in this particular case. It might be that the PST file has some kind of damage. Microsoft suggests trying to fix this by running the Inbox repair tool "scanpst.exe".
This utility can be found in different places depending on your Windows version (32 or 64 bit) and also the Office version:
- 32-bit Windows C:\Program Files\Microsoft Office\Office15
- 64-bit Windows C:\Program Files (x86)\Microsoft Office\Office15
- 64-bit Outlook C:\Program Files\Microsoft Office\Office15
- 32-bit Windows C:\Program Files\Microsoft Office\Office14
- 64-bit Windows C:\Program Files (x86)\Microsoft Office\Office14
- 64-bit Outlook C:\Program Files\Microsoft Office\Office14
- 32-bit Windows C:\Program Files\Microsoft Office\Office12
- 64-bit Windows C:\Program Files (x86)\Microsoft Office\Office12
In the folder matching your Windows / Office version, will be the "scanpst.exe" file. Simply double-click, browse to the PST file and click "Start" (be sure to close Outlook before doing this).
After the tool is finished running, close it down, open Outlook again and repeat the compress procedure outlined above.