Hello ranlp,
It looks like a networking issue between our servers and the source. If it continues to happen, please open a support case so we can have a look.
You can check all the errors using the following commands:
$syncstats = Get-SyncRequestStatistics <your user> -IncludeReport -DiagnosticInfo verbose
$syncstats.Report.Failures | group FailureType | ft -AutoSize Count, Name
$syncstats.Report.Failures | where {$_.FailureType -like "Replace with failure name from above"} | fl
In regards to the doubled size, this is expected since the All Mail folder will be synced into the Archive folder from Cloud.
If you do not want to export that folder, you can exclude it from the IMAP sync from the migration or from GMail by hiding it from Settings->Labels tab-> Uncheck the box "Show in Imap"