Rumble power is seriously overrated, he couldn't even ko Arlovski at heavyweight with all those bombs he landed. And rumble naturally is a lot smaller, he looks like a balloon that's about to explode at 250lbs+.
And his power is mainly from his explosiveness/speed. With all those bodybuilding and how bloated/unnatural he is at his current weight, he probably already lost 50% of his speed and not nearly as explosive as he used to be.Ngannou hit a lot harder, longer reach, bigger naturally, taller, way better chin, better heart and is just as fast. He would ko rumble in the first round without much trouble