Genesis 10:9 says, "He was a mighty hunter before the Lord God; because of this they will say, “As Nimrod was a mighty hunter before the Lord.”
According to the LXX, the greek word γίγας is translated as either as migthy or giant.
Why is mighty used in the context of Gen 10:9 instead of giant?
γίγαντας (gígantas) which in English means Giant(1) in the LXX is used in Numbers 13:33, to refer to the “giants” since Moses recorded the people in the land of Canaan as follows: “and all the people whom we saw in it are men of great stature.” and Moses also says: “and we were like grasshoppers in our own sight, and so we were in their sight.”
This passage in the book of Numbers seems to adequately reflect the word of a “giant” in the LXX rendering, given the context.
As it says in Num 13:33: “…There we saw the giants (the descendants of Anak came from the giants…”
In Genesis 10:9, there seems to be nothing in the context of the passage to lend support to translating it as “giant” as compared with the example from Numbers 13:33.
It would appear that “mighty” is the proper translation here.
For more questions on biblical translation, see: “40 Questions about Bible Translation, by Mark L. Strauss”.
Sources:
The operative adjective used in gen 10:9 is גִּבּוֹר (gibbor) which means "strong, mighty" (Strongs).
As the OP correctly points out, the LXX translates this word (rather surprisingly) as γίγας (gigas) = "giant". The Brenton translation of the LXX translates this literally as:
He was a giant hunter before the Lord God; therefore they say, As Nebrod the giant hunter before the Lord.
Now, why the LXX did not use a closer word in meaning to the Hebrew like μέγας, μεγάλη, μέγα, (= "great") etc, we do not know.
