I think that the Nephilim of Genesis is really just something fanciful made up to offer an explanation for people with different physical characteristics which sounds more mystical and enchanting than any scientific facts.
There are many examples of mythological legends contrived by different cultures in an attempt to “explain” natural phenomena. As an example: according to Hindu legend, the Ganges River in India was supposed to have been created when the water goddess Ganga flowed from the heavens through the long hair of Siva, and in another Indian creation myth, Prajápati weeps creative tears like the Egyptian sun-god Ra, whose rays are tears from which all things spring.
So, we can see how natural things, such as rivers and the sun, were taught to have supposedly originated from tears of mythological gods. Many other things, such as the moon, stars, comets, mountains, oceans, animals, human life, as well as groups of people sharing certain physical similarities, have been attempted to be explained by numerous types of myths. Mythology, of one sort or another, is a common factor in human culture and seems to satisfy some innate and primal need from an anthropological and psychological perspective.
I believe that what was referred to in the Bible as the “Nephilim” was really nothing more than a variety of people called the “Anakim,” who were considered notably tall and strong, at least relative to the Israelites. Facts no doubt became quite exaggerated and distorted, thus adding fodder to the classic “Nephilim” included in the Noachian flood story.
This is yet another example showing that the correct explanation often tends to be the simplest.