Symptoms
After migrating between Microsoft 365 environments, you notice that the overall size is more significant at the destination when verifying your storage metrics, even if the files seem the same size.
Note: This affects any Microsoft 365 migrations, including SharePoint sites, teams, and mailboxes.
Details
The larger destination is expected behavior because Microsoft 365 uses temporary databases to store your data during a migration.
Microsoft 365 automatically clears the data within a few days.
Here are a few things you can try to mitigate any problems with your storage limit:
Delete unnecessary files before your migration.
Increase your storage for the migration.
Split your migration to allow the temporary databases to clear out.