Once you have deleted the mails from your PST file / moved mails from a PST to other PST file, the current PST file size will be remain same. It will occupy the space unnecessarily. We can reduce the PST file size with below procedure.
1. Open your Outlook 2010
2. Right click the PST file which you want to reduce the size
3. Click “Data file properties”
4. Click “Advanced”
5. Cick “Compact now“. It will take some time to complete depends on your PST size.