Legendary foes grow larger with every retelling of the tale of their defeat, but I didn’t realize this was so true of the giants of Greek mythology:
In Greek mythology, the Giants or Gigantes (singular Gigas) were a race of great strength and aggression, though not necessarily of great size, known for the Gigantomachy (Gigantomachia), their battle with the Olympian gods. According to Hesiod, the Giants were the offspring of Gaia (Earth), born from the blood that fell when Uranus (Sky) was castrated by their Titan son Cronus.
Archaic and Classical representations show Gigantes as man-sized hoplites (heavily-armed ancient Greek foot soldiers) fully human in form. Later representations (after c. 380 BC) show Gigantes with snakes for legs. In later traditions, the Giants were often confused with other opponents of the Olympians, particularly the Titans, an earlier generation of large and powerful children of Gaia and Uranus.
The vanquished Giants were said to be buried under volcanos, and to be the cause of volcanic eruptions and earthquakes.
Neanderthals!
The Nephilim might be Neandertals, too, especially since cross breeding is part of the story.