I think BM being stronger than Kaidou is not true at all. BM has so much smaller Skysplitter CoC capacity than Kaidou so that she couldnt even use it in the climax of her battle vs Law-Kidd. While Kaidou used it nonstop since the start of the war, facing 10++ YCs, and didnt go serioys until he fought Skysplitter Luffy for the span of even more additional hours since 1010. Kaidou's CoC is so much higher than BM. Maybe BM has better physical durability, more difficult to wound etc? Maybe. But endurance? Again Kaidou's endurance is astronomically higher than anyone in the manga currently. Regarding Devil Fruit competence, maybe theyre similar in that sense, but other stats Kaidou is significantly higher imo.