To be honest, you guys are putting too much effort in a game about giant girls trying to fuck you. They are living in some sort of period that it's between age of enlightenment and Napoleonic wars, Wich doesn't really make any sense for them to be made by science, as in that time, science was still something very weak (hell, alchemy is probably still a thing in this world)
So in my (stupid, ignorant and unfunny) opinion, there's no way to them being artificial. In this world every race can reproduce with another one, Wich means demons, elfs, giants, furries and humans are probably related in the evolutionary tree, but yeah, this still don't answer how other races can have a giant variation, i am very bad at biology, but maybe in evolution, giants started having problems in this reproduction, and couldn't reproduce with the semen of a "mix" of DNAs and RNAs of other species, so the only species that still has a base DNA and RNA are pure humans ( I don't really need to explain why).
But even my biological theory doesn't make sense, like: why there aren't any giant male, or why they have this wide range of species, but still, cannot reproduce with them.
But in the end, they aren't artificial, as what I learned from Fallout, was that when you try to make a giant human, you don't end up getting a giant hot babe, you get a Behemoth