Hi Jorge,
Which 'R3szchk' patch did you use for the export? I strongly recommend you to use the latest one. I have seen cases where this issue was because 'R3szchk' was calculating the database size wrongly at the export already.
You can download the latest one from
http://service.sap.com/swdc -> Support Packages and Patches -> Browse our Download Catalog
Additional Components -> SAP Kernel ...
Regards,
Hugo