After the migration, the migrated mailboxes are larger than the source NSF databases.
The data could be 1-3 times larger than the original NSF database migrated due to many factors such as the single instancing difference in database/compression technology. In addition, you may see "0" in "View Summaries / User and Resource Details" window.
MNE is trying to read catalog.nsf/properties of each NSF files. MNE's goal is the read as much as it can. If NME cannot read it, it will more likely record a "0". Moreover, the data is only going to be accurate as the last time the "Locate Notes Datastore" is run and if MNE was able to read the data. All these numbers are estimate.
MNE does not get the actual numbers since it would have to open each NSF and manually count the messages. NSF database is usually larger than the amount of data in it since it is a database and holds everything; deleted items are not removed until the database is compacted. Scanning catalog.nsf is the best MNE can do.