Size of in-progress backup differs from the size of backup content
During an ongoing backup, the size of the backup displayed under the progress bar differs from the size of the backup content.
The size of ‘Backup in Progress' is the size of the gradual incremental backup data before it is de-duplicated and uploaded to the cloud whereas the size of the data under Backup content shows the size of data for the previous backup snapshot.
The size of modified data since the last backup determines the size of data under ‘Backup in Progress'.
Other factors such as changes in inSync profile backup shares, which might result in an increase or decrease in the quantity of data backed up, would also play a significant role.