Hi,@Iniobong Nkanga
Thanks for posting your question in the Microsoft Q&A forum.
When migrating mailboxes, there are differences in mailbox storage size, which can be caused by a number of reasons:
1.Sometimes, mailboxes contain hidden items or multiple versions of items that aren't immediately visible in the source mailbox but get counted during the migration.
2.Mailboxes often have Deleted Items and Recoverable Items folders that might not be considered in the initial size calculation. Ensure these folders have been accounted for or cleared out if they are not needed.
Please check the actual capacity of your mailbox first. If the actual capacity of your mailbox is really 257 GB, the specific migration time is related to your network quality.