The giants of Noahs time werent called "giants" because they were big (because it was widely believed that *everyone* before the flood was much larger than humans are now), but rather because they had descended into monstrous degeneracy and were little better than beasts. The post-flood giants were just big people likely still carrying some pre-deluvian genetics, but those genetics probably werent from Cain's race. Also the giants being the progeny of demons is a misinterpretation, demons and humans are physically incapable of mixing because demons are pure spirit and have reproductive organs. The "Sons of God and Daughters of Men" that genesis speaks of when referring to the creation of the Giants is not demons mixing with humans (though there is a second instance where it does refer to demons possessing the giants due to their degeneracy), but rather the Holy Race of Set (called Sons of God for maintaining the true worship of Him) mixing with the wicked race of Cain (called Daughters of Men for their wickedness and predilection for worshipping every demon under the sun.) One of the major sins leading to the flood was at the men of Set's race abandoning the worship of God to race mix with heathens, which is actually why its a commandment of the Catholic Church to avoid mixed marriages with heathens (sadly a commandment that the Bishops have been too afraid to preach, even though it still is in force).