Non Unicode mode limits the PST/OST size to 2GB.
By default Outlook 2003/Outlook 2007 creates PST/OST files in Unicode mode. But if files are created in non unicode mode, quick way to fix this is to change the registry key:
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook subkey.
Add a new string registry value: NewPSTFormat and set it to 2.
NewPSTFormat can have the following values:
Prefer Unicode PST: 0 (default)
Prefer ANSI PST: 1
Enforce Unicode PST: 2
Enforce ANSI PST: 3
Setting the value to 2 will force all new OST/PST files to use the Unicode format.
This will not change the current PST/OST files but will impact the new ones that are created.