1. Is there a biblical support for the term "sons of God" meaning the "sons of Seth?" For example, is there another passage that uses the same reference?
This might be rough for you. Let's actually read what is written.
Here is the lineage from Adam (the son of God), through Seth, down to Noah.
The SONS OF GOD
Gen. 4:26 And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the Lord.
Gen. 5:Genesis 5:1-3
This is the book of the generations of Adam. In the day that God created man, in the likeness of God made he him; Male and female created he them; and blessed them, and called their name Adam, in the day when they were created. And Adam lived an hundred and thirty years, and begat a son in his own likeness, and after his image; and called his name Seth:
Luke 3:38 Which was the son of Enos, which was the son of Seth, which was the son of Adam, which was the son of God.
CAIN "A MAN" (Daughters of men)-unequally yoked
Genesis 4:1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the Lord.
Genesis 6:1-2
And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
Genesis 4:16 And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden.
2. Is there any theory on why descendants of Seth and Cain would yield giant mighty men on a significantly larger scale than the originally created Adam and Eve?
I see the word Giant, but nothing about them being "significantly larger". Maybe you're thinking Rock Hudson was bigger than life in that movie with Liz Taylor. :think: