Based on what? You could fit dozens of mountains in that hole.
You could see Zoro on Pica's torso, does it mean Pica's golem is small? I hope you realise Oda drawing far smaller objects to massive things does not make it less big.
Read above.
No, this is either downplay or you don't know what you are actually talking about. You see the depth of the hollowed part, yet you think this means nothing? Which mountain+ or small island attack is able to create such a DC feat?