Greetings!
The giants being mentioned in the book of Genesis 6:4 are the results of the crossbreeding between the sons of God and the daughters of men.
The sons of God mentioned in Genesis 6:4 are the Angels. These angels left their habitation just because they have thought of the evil of inter-marrying the daughters of men.
These angels according to the above scriptural verse left their first estate which is the spirit to take the human body which they are capable of doing.
2 Peter 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
In the scripture above, this Angels have sinned against God and are known as rebellion angels. They have sinned against God just because of their decisions to inter marry with the daughters of men, a decision that brought about the birth of the giants in Genesis 6:4.
So, because of their decisions to marry to the daughters of men and also have sexual relationship with them, they sinned against God and bore giants.
Genesis 6:4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
In other version of the bible, these giants are called Nephilim and they are known to be different from the normal human that have grown into becoming a giants.
The difference according to Bro. Eli is that, they are strong and might as of old and they are also men of renown which is unlike normal human giants who are not strong and can die anytime.
Thanks for reading.