The total size of backups:
The source data is 50 GB. When its full backup is created, it takes 50 GB of space. Now, next week when 10 more GB of data has been added, the total source data are 60 GB. On creating another full backup, the size comes out to be 60 GB as well and cumulative backup is 50 GB + 60 GB which equates to 110 GB.
In the second week, another 30 GB has been added to the source data, and on full backup creation, the size will be 60 GB + 30 GB which equates to 90 GB. So, the total size of backups saved till date will be 110 GB + 90 GB which equates to 200 GB.