After you remove messages or attachments from your mailbox, the PST file size will not necessarily reduce by itself. In order to shrink the size of the PST file, you need to compact it.
To compact a PST file:
- Right click on the top-most folder of your PST. (It is usually called “Personal Folders”, but may also have a different name).
- Select “Properties for…”
- Click “Advanced”
- Click “Compact Now”