Since the Myrk Giants also have greater average intelligence and willpower than ordinary Krids, I think this is probably the root of their abilities. All Krids, for whatever reason, have great magical aptitude; those who are smart enough become mages and enhance their own bodies, growing into Giants.
That would explain GURPS' vagueness in exactly what makes a Krid become a Giant; the question is the same as asking what makes one member of a species smarter than another, and there's no simple answer to that.
--SiliconDream
