Outlook PST file too large

Some information below to help you resolve the problem - please post back to let us know the results:
Not a personal folders file | MSOutlook.info

The pst-file is on a CD/DVD or other read only medium or is marked as "Read Only"
Outlook needs read/write access to a pst-file in order to connect to it. Therefore you must first copy the pst-file from the CD/DVD back to your local hard drive. I suggest you put the pst-file in a subfolder of your Documents folder and call that folder "Outlook". Make sure you don’t overwrite any existing pst-files.
After copying right click the pst-file and choose Properties. Here verify that the "Read-only" file attribute is not selected.

Don’t import! Don’t import! Don’t import!
Am I clear? Don’t import! Once you’ve restored the pst-file via the above method don’t use import to make the pst-file available in Outlook. The proper method to open a pst-file is to connect to it via File-> Open-> Outlook Data File…

Use scanpst.exe to scan it for errors
Still no luck? Then use use scanpst.exe to check the pst-file for errors. The location of scanpst.exe depends on your version of Outlook and Windows. See this post to find the location for your version.

Use a 3rd party recovery tool
When scanpst.exe is unable to fix the pst-file there might have been some serious corruption in the pst-file. There are some 3rd party recovery tools available like OutlookFIX. These tools are specialized to try to recover every single recoverable item from your pst-file. Be warned; these tools don’t come cheap. However, depending on how much the lost emails are worth to you, you might consider it a bargain after all.

Finally, should you ever need to reduce the size of your files, you can compress them:
How to compact PST and OST files to eliminate deleted item space in Outlook