This can easily be corrected in two ways.
You can use the case.prefs file to either:
A) change the limit to higher value such as 100MBs using this: ExportPstMaxItemMetadataSizeMB=100
B) Or you can disable it completely by using this: ExportPstSkipLargeItemsEnabled=False.
More details can be found in the user manuals under the "PST Options" headings.