Answer:
Gigantism (and it's opposite, dwarfism) can occur in any isolated environment. We call it insular gigantism or insular dwarfism (although it doesn't always happen on an island, just in an area that is island like, such as deep underwater and in the middle of the jungle). This is how we think the Pygmies evolved in the middle of the jungle. There is an island called Flores in Indonesia. Here we can find giant rats and other things that are way bigger than their relatives everywhere else. In the past on that island many things were smaller than they should be instead of bigger. We even found remnants of an early humanoid that was much shorter than us but fairly close to our intelligence (according to the size of their Brodmann's area 10). I think we named it homo florenses. So, it turns out that whenever you isolate an ecosystem, everything in that ecosystem may evolve to be smaller or larger.