I can't recall seeing it in the literature, but I think another reason for the longer hook might have been to counter-balance the weight of the wings. I've seen alot of Wulff's (my own and others) tied on standard dry fly hooks that tend to land on their face, especially after being fished for a while and picking up some water. Another excellent hook, and BTW the one that Jack Dennis often uses for Humpies, is a Dia Riki 300. It's about 1 eye-length longer than a standard dry fly; sort of between a 2X and 1X shank.
Merry Christmas to all!
Arnie