The following is an excerpt from the MNE Administration Guide, on page 12:
About MFNE statistics
MFNE’s Notes Migration Manager and several of the MFNE wizards report data statistics from the Notes
environment—sizes of message stores, data volumes migrated, and so forth. Note that the statistics, and
corresponding throughput, reported by Migrator for Notes to Exchange are based on uncompressed data sizes
provided by the Lotus/IBM APIs. The APIs extract the data in a compressed format and subsequently provide the
data to MFNE in an uncompressed state. As a result, the data amounts reported by MFNE will likely be higher
than the numbers observed natively in Notes or the volumes transferred over the network to the migration
servers.
