Resolution:
With regard to disk usage,  the space used on disk is not only for application messages. There are possibly other internal records, such
as transaction, acknowledge records, etc... Also, EMS stores records on disk by blocks of 512 bytes. As an illustration of the worst
case scenario, if the total size of a message (not just the payload) is just above 512 bytes, the space needed on disk is actually doubled
as we store the data as blocks. So disk usage will be higher than the message record size because of the internal records.