NOBU Backup Size Allocation

I have 5 computers with backup data using Norton Online Backup. It shows 40% of 75GB. I want to know the breakup of space allocation (which files and computers are using this space & how much is consumed individually by them) so I can manage the same. It only shows the last backuped up data size. Is this the total size consumed by that particular computer?

Also I want to understand if one set of 25GB data subscription expires (if i dont renew the same)  and if buy another set of 25 GB data. (coz in some cases buying a new data set is cheaper than the renewal!) What happens to my total data space and existing data stored onto NOBU?

Thanks & regards