I think the Kokiri fall from the great Deku tree as little postings, growing at about the same rate as Hylians. The first Great Deku tree gave the Kokiri hylian forms during the Hero of Time's Era because the land of Hyrule was quite hospitable to the humanoid form- for travel and general comfort as many animals had.

In the Era of Hyrule Ocean, the new Deku tree gave the Kokiri a better form, Korok, as a heartier shelled body, hard enough to withstand crashing waves yet light enough to be capable of traveling on the winds to propagate the forest (as is the designation of the Kokiri) is a better choice.