Difference in folder size and PST file size

I am using Outlook 2010 with Windows 7 Enterprise Service Pack1. I have one Outlook PST named with Personal Folders.When I open MS Outlook 2010 application / right clicking on my "Personal Folders" - Properties/ and after click on Folder Size - the file size shows a healthy File Size of only 439,404 KB.

However the Outlook PST file located in C: > Users >username >My Documents shows a File size of 9,049,041 KB.

Why this size differance is happening?

I already tried to compact and export to another PST. But still the issue is persist.

Regards...