I'd say it's mostly due the casting wall thickness, pretty much cut in half from 5/16" to 5/32". When GM was designing the all-new Chevy and Pontiac V8s for 1955 this was part of the weight savings campaign, and a new revolution in casting techniques was major part of it.
By the time the sixes were due for modernization in 1962, the same casting advances were applied. The older designs which were taller (unable to fit under the low hoods of newer bodies) and longer and had thicker block and head castings had to go. The new thin-wall engines which shared some components with the small-block V8s (one more step towards corporate engine sharing) were 24% lighter than the old designs.
It's not hard to see that at least an extra hundred or so pounds of iron could be saved with the thinner block and head castings.
