Backup takes way too much space

I have been letting N 360 manage my back-ups for over three years, but now I get error messages becuse my backup drive is full at about 460 GB. 

 

The c: drive I am backing up has lots of space - all my applications and files only take up about 148 GB.  How can the backup (freshly run today) be so much bigger than the data it is backing up?

 

I figured the answer must be that previous back-ups are there too, and I need to delete some of them.  But when I go into "Manage Backups" I find there is only one set of files ("Default Set") and that cannot be deleted!

 

I could take a chance, and try to just delete the whole thing with Explorer, and start again, and hope it will be smaller.  But that does not seem like a long-term fix, as I have not changed anything to prevent it growing out of all proportion again.

 

Please, any ideas on how to set "backup" to just back up what needs to be backed-up each time, without taking more space than the original files need?

 

Cheers,

John