When you re-register VMs in the inventory (by adding it to the vCenter Server or switching from direct host to vCenter Server connection), these VMs are treated as new ones, that's why you see a spike on the backup size when backing up VMs. Please search these forums for more info on this behavior.
The correct way would be to re-add these VMs to the job, re-add them in the backup copy job and your job will continue running as they did. Also, you may consider using backup mapping functionality > Step 6. Specify Backup Storage Settings
(should be used when VM IDs remained intact).