N360 Backup fills all available space

Over time N360 Backup fills the entire networked backup drive I use for back up. I have two computers using N360 backup. The combined amount of data being backed up is about 250G. The space available on the backup drive is about 700G. Over a number of months the backed up data will grow until it fills the entire space available on the backup drive. How do I keep the backed up data on the backup drive from growing beyond the size of the data on the PC's. What I have done in the past is delete the backed data every few months and start with a new backup. Is that the only answer or is there a way to keep the backup data from getting out of hand in the first place. Thanks. Mike