It is because it is a full blown OS that runs off of that image file, so it needs free space just like the OS does on your Macs. You can trim the nbi file >down a lot. There was a post on this mailing list a while ago that lists all non essential items you can delete. For example, I delete every single app out >of /Applications and /Applications/Utilities except for Disk Utility. All other apps I don't need. You can get rid of fonts and most app support files in >/Library.
I have my nbi file trimmed down to about 7 gigs now and keep trimming off the excess every time I create a new one.