The existence of something like King Kong on an island is unlikely, and here’s why.
There exists something in nature known as island dwarfism, in which animals found on islands tend to be smaller than their recent ancestors who live on mainlands. This is because of the limited resources available to those living on an island.
The environment of the island selects for the members of a species population who require less calories, since less calories are available. Food availability will fluctuate, clearing out the larger members of a population.
That said, unless King Kong was living off of sunlight, I don’t know where in the world he would get his 10 billion relative calorie count.