What exactly are you backing up?
A system backup takes up a lot less space.
If your backing up your data from the User folders then more is required.
Personally I don't trust windows back ups - as system restore, reset keeping data, reset not keeping data, and system back up all let me down after the Anniversary update last year. Only a system image created using Macrium reflect saved the day.
You would be better saving back ups of any description to a external drive.