Explaining Kong's size (or trying to)
My best theory on Kong's size is the following:
Growing up, he had to survive and compete every day of his life against skullcrawlers and other nightmarish creatures in skull island, putting a lot of stress into such a young kong, which stalled his growth, once ramarak (Kong's biggest danger) was defeated and probably discouraged most skullcrawlers, he had a lot more resources and room to grow.
Being an ape and thus related to humans, it also means he could have several growth spurts on his late teens (post 1973 where he was stated to be "still growing") like we do.
All that combined with the fact he's an alpha specimen of his species, (proof being all other members that were under skar king were smaller than him and ignoring Ghidorah's alpha call)
And with alpha members tending to be bigger than the average managed to make kong the size he is currently.
Of course none of this is official, but I like the way I tied it up.