Continuing my observation from http://community.norton.com/t5/Norton-Internet-Security-Norton/Problem-with-generation-update-delta-for-virscana-dat/m-p/815990 I can assume that indeed Norton has some problems with generating update delta for virus definitions using complete set of virus definitions (i.e. Smart definitions option is turned off).
Here is a picture of the standard virus definitions update content, in which you can see that delta for virscana.dat is normally generated when using smart definitions, but when using full definitions set, delta for virscana.data is not generated. Instead of copying only changed parts of the file, the file is fully copied into update package which means that update size will be significantly larger.
PS
The full size of virscana.dat is 4 479 786 bytes long(around 4MB 375KB) while delta size is (as you can see in the picture) 139 bytes long. Which means that 3 438 226 bytes(3MB 358KB) of Live Update size is just only compressed virscana.dat