Windows 7 is just like windows vista. The install image contains all the versions of windows on one disc, and your cd or computer key is the actual thing that tells windows what version you are supposed to have. Thus some things would not be installed and others would. However with all this in mind, the actual image should not be more then 3.6ish gigs for windows and even Dells stuff shouldn't take more then a gig or so unless you got a lot of junk. All that not withstanding, have you tried to do a DVD to DVD copy using the burning software that came with your computer? It is possible that if you ripped the software from the dvd, it uncompressed something so your image got bigger and bigger.
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If you do a bit for bit dvd copy, and they used a normal dvd (not a DL) then you should have no problems with the backup. I HIGHLY doubt Dell would send you a dual layer dvd recovery disk since they are pricy even for manufactures. I had something else but i forgot it. Hope this helps!