Kaido was strong like Kyros, Jinbei, Wiper or Oden. Extremely strong for his island. Not extremely strong on a global scale. Then he traveled, got stronger but still was in need of Big Mom gifting him an OP DF to reach the next level.
Even then, at 39 yr old, he went life and death with Oden. And he still didn't reach his prime at that moment because he missed leg day.
Big Mom on the other hand was born a monster.