We actually have todays episode of one piece in it we see sometimes where momonosuke can fit in a single room inside onigashima and then we have something like this
We have times where kaidou and momonosuke fight
Where both Dragons seem almost irrelevant to the size of onigashima nad then this
Where momonosuke is comparable to the complete island
We have this were king seems comparable to the same 3km horn
As you can see the size changes a lot throught the multiple fights also im pretty sure they opened multiple holes though the same side of the skull of onigashima which doesnt make any sense