The way I see it is that they are floating from Magma. Kaido has a flat body, so you would see his whole body while BM has round body so you could only see part of her body( like iceberg). That's why BM has smaller appearance than Kaido.
We'll see in a few chapters from this one, then. I look forward to the reveal of her fate after this arc. And I also look forward to her crew's plot point being resolved. They have her vivre card, last time they check it was before their ship being knocked down the 2nd times by Marco in 981.
For conclusion, this is the actual height comparison. She is literally the tallest Yonkou.
advCOC making you part of the best =/= automatically making you above anyone who don't have it. FS make you part of the mightiest stated by Rayleigh (who isn't any reliable than Kaido), is Katakuri above anyone who don't have FS?