Rashi explains that Moshe was afraid of the Zechus that Og had, when he served Avraham by informing him of Lot’s capture. We have two stories of Tzadikim who killed giants. Here we have Moshe who killed Og, and in Sefer Shmuel we have Dovid killing Golyas. There is no doubt that conquering Og was a sign of the Gadlus of Moshe who was Kovesh (conquered) his Yetzer more than Gantz Klal Yisroel, and Chazal say that Moshe was: שקול כנגד כל ישראל equal to the entire Klal Yisroel. He was a “giant” in his unusual K’vishas HaYetzer (conquering his desires), so he was the only Yachid in Klal Yisroel who was: ראוי worthy to kill a giant.
And Dovid was the Yachid who was: משכמו ומעלה מכל ישראל head and shoulders above everyone else, since he was a Melech who is higher than everyone, because he had an unusual Yetzer Hara, being an Admoni (flaming red head) similar to Eisav. That’s why only Dovid, with his unusual Gevura in his K’vishas HaYetzer, was: ראוי worthy to kill the giant Golyas. Perhaps the giants were big and powerful because they also had greatness in Ruchaniyus, and this is why Moshe was afraid of Golyas. Indeed, Chazal say that Orpah was Zoche to 4 sons who were giants because she was good to Naomi. But when she turned her back on Naomi, she lost everything.
Despite All Your Great Ruchaniyus Potential, You Can Lose Everything by Following Your Ta'avos
She had great Ruchaniyus'dik potential, but the Ta'avos of Olam HaZeh ruined her, and as soon as she left Naomi, she was Nichshal with 100 men. (Gemara). And the same with Og who helped Avraham Avinu; but he spoiled everything when he wanted Avraham Avinu to get killed so he could marry Sara.
