Devarim 3:11 desribes Og's bed as being made from iron. What does the the Torah add by describing Og's bed as being made from iron? The Ramban comments that it demonstrates how tall he was, that wood would not have been strong enough. But surely a bed could be made of wood with the appropriate construction to carry any amount of weight. Also, being tall does necessarily equate to being heavy.
The Rashbam explains that due to his strength he broke the wooden beds when he was younger and therefore they made a metal bed for him.