The size of a SP is variable. For an offline installer, is this x86-32, x86-64, IA-64, or an ISO that has a combination of the three? Does it contain every language? Or just a few? And the SP delivered by WU is significantly smaller because, not only do you download only on architecture and one language, it also uses differential compression (i.e., it works more like a patch file of the olden days).