Outlook .pst file

I have a client that uses outlook 2007, his current .pst file is 21gigs.
He has and old laptop and it's giving his problems, he got a new laptop.

I was unable to export his .pst just continues to cut off (stop) at around 8gig to 16gig. I was also not able to do a repair on the .pst file, the laptop just freezes on me.
I went into the outlook folder and coped and paste his .pst file to and external hard drive. ( only solutions I could thing of at that time)
I re imported it into a spare laptop by copying and pasting the .pst file into the user\document\outlook folder.
I created his new account and pointed it to that .pst file.
when I copied and paste it, it showed 20.9 gigs, but now it shows 14.9 gigs.
due to the large size of the file my client can't really be sure if everything is there or not, while it seems to be there he may look for something in a few week that dates back a year and he may find that it is not there.

The laptop that is currently installed on is a spare laptop that he was going to sue until the new laptop was shipped, but he never used it.

So I can't explain how the .pst file went from 20.9 gigs to 14.9 gigs without him even using the laptop, the only diff in the the .pst file was on outlook 2007 and now he is using outlook 2013.
Can anyone explain the difference in .pst file size? How dose it go from 20.9 gigs to 14.9 gigs without anyone using it?
Dose outlook 2013 compress the .pst file?
I'm planing on calling Microsoft tomorrow, but in the mean time I was wondering if anyone could help.
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I am always compressing my PST / OST files and archive by year.

But this article should be a start: Managing PST Files in Microsoft Outlook
noadAuthor Commented:
The question is not how to do it, but why it went from 20.9gig to 14.9 gigs on it own...
Any idea how this could occur?
Lee W, MVPTechnology and Business Process AdvisorCommented:
PST files don't compress/shrink as you delete data - they only grow unless you MANUALLY shrink them.  So if the file WAS one size and now it's smaller, then it's probably just fine.

I'd just ask why you bothered exporting.  Just copy the file.  It's a PST file and is used by both Outlook 2007 and 2013.
noadAuthor Commented:
I did copy the file to its new location on the doc folder on outlook 2013, but not one used that laptop and it went from 20.9 gig to 14.9 gigs.
Can you explain that?
I can't...
Any suggestions?
JohnBusiness Consultant (Owner)Commented:
Two things:

1. As noted, compacting the file (you moved it and opened it in a new Outlook) could compress it by that much. It seems a lot but likely it has never been compacted.

2. Use the SCANPST.EXE from Outlook 2013 and try again repairing the original file. It might work.

Finally, keep the old file in a SAFE and KNOWN location and move on. If you / they determine something is missing, then you have the file to try again or use a commercial program to retrieve the data.

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