I've been thinking lately about the design of Onigashima... and I think Oda's reasoning for making its SO HUUUGE... that the ships look SO TINY in comparison is for Oda to draw nice feats of Slicing/destroying such LARGE structure through a move...
Like for example... take the mountains for example... there are SO MANY MOUNTAINS... that Oda can use it for various fights to showcase the destructive capacity for a battle... but having that mountain being sliced... and here on this island... it FAR EXCEEDS things like "Punk Hazard mountain (Law) /Pica gigantic statue (Zoro) /Marineford Tsunami iced mountain (Mihawk)"
I honestly think that only ONE SINGLE MOUNTAIN looks bigger than any of the three mentioned above. And there are PLENTY of those mountains... So I feel Oda will make usre to use their size to display the power of specific characters
Also.... Another TWO GREAT SPOTS I have see is... the TWO HORNS of the skull.... those two horns LOOK HUUUGE... I can see TWO BATTLES on each horn... and I can see at least the destruction of one of the horns... maybe by being sliced.. or by being destroyed in powerful normal attacks