Migrating 6.0.10 32 Bits to 8.0 64 Bits

Hello,

We are trying to migrate to the latest version of zimbra open.
We are using Ubuntu 8.04LTS 32bits and the new server is Ubutu 12.04LTS

By using the migration tool, the backup completes successfully, backup and restore all happens without errors. But note that the size of the mailboxes on the new server take up far less. The large mailboxes, over 6 Gb observed with a size difference in origin and destination of about 150-200 Mb

Example.
Old Server mailbox: 13520 Mb
New Server mailbox: 13280 Mb

Making e-access tests, we don't see emails lost, but we dare not make the change because the new mailboxes may not have all the information.
Apparently all emails have been migrated, but the size does not match

Any way to check?

thanks