Looks are often very deceiving. With carbon nanotubes, the dimensions you provided would be plenty strong.
Except that it's not a solid structure. It's hollow, with pipes and elevator tubes and laboratories and fuel tanks and observatories, and all kinds of other things.
And initially, it was built in a hurry in Norwegian shipyards out of our specified steel.
We've been strengthening it since, with our carbon nanotube factories, but at first, it was just brute force engineering.