A new PST file format was introduced with Outlook 2003, which doesn’t have the 2 GB size limit that the previous format had.
However, the PST file format is not automatically converted when you import an old PST file into a newer version of Outlook, and there is no way to directly ‘convert’ one file to the new format. The easiest way is to create a new file and then move/import the messages to it.
Take a look at this:
<a href=”http://office.microsoft.com/en-ca/outlook-help/increase-the-storage-capacity-of-your-data-file-HP001038352.aspx”>Increase the storage capacity of your data file</a>