Big Nog only lost once to a fighter who was smaller than him. That was Dan Henderson, which was 7 years ago. Other than that he has only lost to Fedor and Josh Barnett, and he constantly dominates other elite heavyweights. I really don't see him losing to a smaller opponent at this point in his fighting career. Besides, if Babalu could submit Shogun, I don't think Big Nog would have any trouble.
Shogun's a great fighter, but Big Nog is on a different level.