As I said, if you're not so sure about this, you sure can wait until we actually see the comparison between Onigashima/Flower Capital and this hole.
Btw, I read a post here mentioning how Onigashima could be placed on this hole later on after Kaido is defeated and the danger being gone.
Also, I said the hole is comparable in size, not exactly as large as Onigashima, there's a difference. My initial argument isn't also the surface level, although the hole is quite large as I illustrated above, but the actual depth of the hole.