If you can figure out what software you are using there is probably a place where you can set a retention period or incremental type backups.
if you do need the historical copies you'll need more drive space.
Before I go suggesting you buy more drives to increase storage, make sure you are only backing up what you need. If you have installed applications, you probably don't need to back them up, you can always reinstall.
Assuming you are only backing up what you absolutely need (i.e. your documents, pictures, etc) and you need that much space per backup, you may be better buying a NAS box and running backups from it.