You launch a Copy mailboxes operation and get the following error in your migration report:

The message exceeds the maximum supported size. Cannot save changes made to an item to store.

This error usually appears when the message you are trying to copy is too big for the destination.

This can happen if the message limit for the destination is smaller than the source's.

You can solve this problem by changing the destination's message limit during the migration to match or exceed the source's.

Once you finish your migration, you can reduce that limit to its initial value.

To learn more about the limits, see Microsoft's docs on Exchange Online limits.