By using of compact PST file option, only 20% space of total size of PST file is reduced. After compacting PST file 5% of “white space” is left in PST file, which is required for performance reasons.
You can also try truncation tool to reduce size of PST file at least 20 to 30MB PST data easily. This utility is free and you can download it from here: http://www.splitpst.msoutlooktools.com/how-to-reduce-the-size-of-outlook-pst-file.html
You can also use MS Outlook archive option to reduce MS Outlook PST file size. By the using this option you can easily manage large PST file into small size and secure older outlook data into hard drive as your define location.
I hope it will be helpful for you.
Thanks & Regards