This might work...
I suspect that your PST file needs compacted. That is, where your file has expanded with many different messages over time, and you may have even deleted a lot of them. But I have seen where the space of those now gone messages is not automatically returned to your free disk space on your drive.
To compact a PST
1) do a right-click over the top-most folder, which is probably call something like "Outlook Today ..."
(If it is a PST that you've imported, then while the name may be different, it will still have some top-most folder.)
2) Select "Properties"
3) On the General tab, select the [Advanced] button.
4) Then click the [Compact Now] button.
Since it sounds like you haven't done this in a while, if ever, expect this to take a while.
Hope that helps.
The compaction, should not affect any existing objects in your PST. It is only supposed to reduce the wasted space, but your mileage may vary (grin).
Got a Outlook2002 pst that is 1.2 gigs but when I load it in outlook, it is empty. How can I get these files out and into the other outlook pst file. Tried importing and used SCANPST.exe.