An increase in the original message size may be noted when migrating content from one Enterprise Vault location to another.
a) Message size in source EV Archive
b) Message size in target EV archive
Enterprise Vault (EV) records the message size originally noted from any item in its source. When the content is archived directly from Exchange, the item within the mailbox reports a smaller original message size then when the same message is written out to disk.
During the EV to EV migration process, items are written to disk from the source and than taken from disk to be written to the target. This action results in the size of the message reported in the target system to align with the size of the message on disk, and not the original size recorded in EV when the item was archived directly from a mailbox.
You can check the source item size on disk by downloading original item from EV.
Original item downloaded from EV: