In Outlook 2003 and 2007 the maximum recommended size of a Unicode pst-file and ost-file has been limited to 20 gb
In Outlook 2010, 2013 and 2016, the maximum recommended limit has been set to 50 gb
you can increase the limit with Group Polices and Registry
Registry
Outlook 2003
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\PST
Outlook 2007
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST
Outlook 2010
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\PST
Outlook 2013
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST
Outlook 2016
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
create new DWORD with value MaxLargeFileSize
Don’t set this higher than 4294967295 (decimal) or ffffffff (hexadecimal)
Group Polices
User Configuration-> Administrative Templates-> Microsoft Outlook <version>-> Miscellaneous-> PST Settings