How does the attachment size filter work?
Notes Migrator for Exchange (NME) retrieves all attachments from the Notes document using the Notes C API call MailGetAttachmentInfo(). A size difference may be seen (for the attachment) as NME retrieves the size of the attachment after it has been extracted to the local filesystem (where NME is installed).
It was found that the MailGetAttachmentInfo() is not reliable for reading the actual size of the attachment. So NME always extracts the attachment to the filesystem (where NME is installed), then obtains the size from the Operating System.