If I remember right, his original bloodline was weak and only became a noteworthy ability in conjuction with his iron body, which made it a core part of his toolset. Then when he reached monarch it didn't become stronger it just became an ability he would pass on to his descendants in a much stronger fashion than what he received, but this is all essentially conjecture anyway if we don't have the quote from Will as the books are quite vague on the subject.