According to Biblical account, why fallen angels' offspring are giants? Or are they egg-born?

Question

In the epic of Sodom and Gomorrah the angels appeared to Abraham were comely men desired by the citizens there, that is, they appeared as normal humans to the people except they were extraordinarily beautiful. Thus Lot was able to exchange them with his daughters. Second, if the fallen angels were giant in size they were impossible to have sexual intercourse with human women. Last, if the offspring from the fallen angels were giants in size possibly impossible for the human women to give birth to. Therefore from these analogies the angels, thus the fallen angels, are in human size, not giant size. Why their offspring were giants?

Further, all angels are male because the Bible only said that they took the daughters of man for wives, none mentioned some took the sons for husbands, is it?

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Discerning suggestions:

• angel has power of transfiguration

I don't remember mentioned in the Old + New Testament unless if there other non-Biblical texts. However, Satan in Eden appeared as a serpent. But not certain if the Serpent and Lucifer are the same, or teaching derived by the church to interpret Bible? The Bible doesn't state directly the fallen archangel Lucifer is Satan.

• could the giants be born as eggs first?

(I know this is absurd and too imaginative e.e). If Satan the Serpent is Lucifer, it follows that the offspring of a snake will be egg-born. Interesting is the corresponding accounts of Helen of Troy and Castor were born from egg, Zeus morphed as a swan and had intercourse with their mother, Leda.

• If the giants be born as eggs...

Similar to some insects going through gradual metamorphosis, as the nymphs exuviate during growth. This make possible the infant and adult size varied extremely compared to viviparity.

Answer 1

There is here and there some confusions in your question.

The Nephilims

What your are talking about in your question are the "Nephilims". Let's read the Bible (I will use the NIV as a reference text).

6-1: When human beings began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose

6-4: The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.

Notce that: the text imply the "sons of god" (not fallen angels) got children with "female human beings" (clearly). The text does not imply the resulting offsprings ARE giants in size.
The "Son of gods" is bene heloim in Hebrew, bene Heloim does not actually refer to "angels", fact is in modern judaism it does NOT at ALL. A certain number of late Hebrews/early christians commentors translated Bene heloim by fallen angel. Reason is their resulting offsrpings, the "Nephilims", is probably in Hebrew coming from "N-PH-L[naphal]" to fall.

Later on in the Numbers we find that:

13-3 We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them."

Nephilim is translated either by "Nephilim" or "giants". It comes from the hebraic "hannəppilim" which has no direct translation.

The Bene Heloim

We find references on the "Sons of god" in biblical passages:

Job 1: 6 One day the angels[Bene Heloim] came to present themselves before the Lord, and Satan also came with them.

Job 2-1 On another day the angels[Bene Heloim] came to present themselves before the Lord, and Satan also came with them to present himself before him.

We see here the "Sons of gods" linked with Satan (the adversary).

Luke 20 :36 and they can no longer die; for they are like the angels. They are God’s children [remind Luke is written in Greek], since they are children of the resurrection

Here, and everywhere, Jesus refers to the son of gods as human beings, without a doubt, they are "like angels", which fairly clearly imply they are NOT angels, and certainly not a fallen specie of some angels.

So the notions of the meaning of the bene heloim is not quite easy to understand. The NIV clearly use "angels", systematically. You can also find, depending the translatin, Sons of God. Heavenly beings.
You will find those "play with the translation" in most language. The sense to give to "bene heloim" is totally unclear.
Let's add that Job, which i quite used here totally know a term in Hebrew for "angel", as shown here:

4-18 If God places no trust in his servants, if he charges his angels [ube malakaw] with error,

Kalm malak, malakaw in Hebrew meaning sending a messenger, in Greek an "Angelos". Here are your angels.

Conclusion

• The Nephilims are giants: the Bible never explicitly mentions them as... Contrary to Goliath which is NOT a Nephilim, he is a Rephaim, common foes of those time Hebrews, and clearly depicted as a giant (His height was six cubits and a span ==> around 3m, when the Bible is mentioning giants, the Bible states their heights...). So EVEN if they was giants, this is not particulary a trait from them.

• the Nephilims are the offspring of fallen angels: the term sons of gods in the Genesis is quite unclear; they could be angels, fallen angels, or humans one thing is clear! they are male... because they fall in love with the daughters of men. Fact is, they could perfectly be extraterrestrials or visitors from the future.

• the Nephilims are highly praised (They were the heroes of old, men of renown) it seems pretty strange to praise the sons of random falty fallen angels of your God and seemingly survived the Flood as the rest of mankind (in those days—and also afterward)

As a lot of things here and there (Example: Excalibur) there is a traditional common culture which is linking the Nephilims as the giant offsprings of fallen angels and women. Except the Bible, while not stating this interpretation as false, neither specifically hint it as exact. The translation of Nephilim as giants and sons of god as angels is "personal".
the only thing I can do so, is debunking the extraordinary false conception.

Answer 2

[I]f the fallen angels were giant in size they were impossible to have sexual intercourse with human women

[A]ll angels are male because the Bible only said that they took the daughters of man for wives, none mentioned some took the sons for husbands

The angels were able to have sexual intercourse with human women because they took on the form of a human. Angels actually have no gender, so they could make themselves look however they want. And they don't always look like people; many Christians believe that the serpent in the Garden of Eden (Genesis 3) was actually Satan himself, who is the leader of all fallen angels. They could also be whatever size they wish; if they wished to be giant, they could be giant. So that they could have sexual intercourse with human women, they would have made themselves look like very charming and handsome men who were about the same size as the current men, although they very well could have been slightly taller.

The offspring of the fallen angels resulted in people who basically were legendary. I personally believe that these people are what most myths in the ancient world are about, of course the stories would have been slightly altered with time. The fallen angels likely had the ability to alter their DNA according to how they wanted it, allowing them to produce legend worthy people.