Microsoft Office Outlook 2007 & Outlook 2003 have both a different format and a larger overall size limit for the personal folders (.pst) file than the .pst files that are in the earlier versions of Microsoft Outlook.
In Outlook 2002 and earlier, the .pst files are in the American National Standards Institute (ANSI) format, and the overall size has a limit of 2 gigabytes (GB).
By default, .pst files are in the Unicode format in Outlook 2007 and in Outlook 2003. Additionally, the overall size of the .pst files has a limit that is more than the 2-GB limit that is imposed by the ANSI .pst files. By default, the limit for a Unicode .pst file in Outlook 2007 and in Outlook 2003 is configured to be 20 GB.
As in earlier versions of Outlook, Outlook 2003 Internet Message Access Protocol Version 4rev1 (IMAP4) accounts and HTTP accounts use .pst files that do not use the Unicode format. Therefore, the .pst files for IMAP or HTTP accounts in Outlook 2003 are limited to 2 GB. In Outlook 2007, the Internet Message Access Protocol Version 4rev1 (IMAP4) accounts and HTTP accounts do use Unicode format .pst files and are not limited to 2 GB.