Space Required for Back up

I am using the back up in Norton Security to back up to a NAS drive on my local network.

I have noticed that the space taken up by the back up is significantly more than the files in the back up set. For example the back up set from my PC (according to the back up preview) is 32,000 files - 43gb. However the space taken by the back up set on the NAS is 98,000 files and 135gb space.

Does Norton Security store more than one version of a file? So if I have a file that gets modified every week are all the old versions stored in the back up or only the latest version? Also when I delete a file on my PC does it also remove the file from the back up on the next back up operation? 

If this is the case is there a way to change this behavior so that the back up set only stores a copy of the current files on the PC?

Sorry for the simple questions but I couldn't find a detailed technical description of what back up does in the support files.