Are you using outlook 2002? Actually this type of problem generally appears in outlook 2002 and earlier version. You should not worry about this problem as it happens due to inbuilt size limitation of outlook. You should use service pack released by Microsoft to solve this issue. OR it is better to convert PST file in unicode format that is available for outlook 2003 and later versions.
However, if these steps are unable to solve the issue then use third party <a href="
http://www.outlookpst-repair.com">outlook pst repair</a> tool to solve the issue.
Try this, all the best!!!!!!!!!!!!