If performing a migrate export on a R77.30 SMS, the big driver of how large the export file will be is the number of Database Revisions that are present, especially if revisions are being automatically taken every time a policy is installed. Try to clean these up as much as possible on your R77.30 SMS prior to the export.
For an R80+ SMS, the main two elements that will significantly increase the export file size are the presence of numerous old IPS update packages, and old sessions that should be purged. Once again cleaning these up will significantly reduce export file size.
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