Giants in what way?The word “giants” (KJV), means “to fall upon as an oppressor.” “Giants” does not mean big in the sense we use the word.
Sons of God were righteous men who worshiped and walked with God. If you look at where they came from , it was the linage of Seth, not Cain.
They married ungodly, unbelieving women (the daughters of men)Those in authority ( giants of Godliness) before the flood that should have but did not discourage ungodly conduct, actually advanced wickedness.(became giants of ungodliness)
What kind of children is produced when the sons of God marry the daughters of men? The kind that will not walk with God because of the influence of the unrighteous mate.
The nephilim were notable men of the pre-flood world who, rather than leading their generation in godly behavior, contributed to its moral downfall. In spite of their prominence or size, their wickedness, violence,and corrupt behavior led to their ultimate destruction in the flood. They were not monsters, but great men who were also ultimately brought down.