I saw that Rabbi David Zvi Hoffmann says what I was thinking 🙂
Simply put, the bed was large not because he was necessarily that tall, but because he ordered them to make him a large bed for honor, like the walls in a city.
(By the way, nowadays couples love large beds; that’s what I heard from a furniture-store owner, who said that today they sometimes have to break down doors to get the new mattresses inside.)

“Nine cubits, etc. — approximately a length of four and a half meters and a width of two meters. Clericus conjectures that Og intentionally ordered a bed made larger than necessary, in order to appear even more gigantic in the eyes of his contemporaries and of later generations, and he points to a similar case involving Alexander the Great, as reported by Diodorus Siculus 95, 17. If so, one who finds it difficult to imagine such gigantic height may explain the matter according to Clericus’s view.”

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By the way, there are those who explain aras not as a bed. Look in the commentators.

I don’t understand what is impossible. There is evidence for sauropods over 40 meters long. If they had not been discovered, people would claim that such a creature could not bear its own weight (that is in fact what people thought at the beginning of the research). A person four meters tall is not some legendary monster. The book of Genesis refers to the existence of the Nephilim as a fact, quite casually. There is no reason at all to interpret this non-literally.

I think the description in the Torah proves the authenticity of the description.
The sages and commentators already wondered about his body proportions, since his width was close to half his length (a length of nine cubits and a width of four cubits); see Devarim Rabbah, where they wrote that he was ugly, and the Alshikh’s commentary, which is puzzled by these measurements.
In practice, scientifically speaking, there are many physiological problems for a person of such height,
and even the “giants” known in our own generations reached only around 2.70 meters, and suffered from health problems and lack of stability. And they certainly could not serve as a mighty warrior on the battlefield as Og did.

But it turns out that the fact that his body width was around 4 cubits largely solves those problems. It greatly improves stability and may solve other physiological problems as well, but this is not the place to elaborate.

In other words, it is unlikely that an “anonymous writer” in an ancient period simply decided on his own to give Og strange proportions, and it is also hard to assume that he was aware of the physiological problems such height creates and intended to solve them by adding width. The most reasonable explanation is that this is an authentic description.

If we add to this that the measurements can be reduced — for example, by saying that a cubit is 5 handbreadths, that a fingerbreadth is 1.9 cm, and by reducing to two-thirds of the bed’s size in accordance with Maimonides’ words — then his height can be brought down to about 2.3 meters, something that does exist in our generation. In that case, of course, the great width greatly improves stability and solves most if not all of the problems.