How is David Robinson so built but only weigh 235?

I was looking up David Robinson and he only weighs 235. Everyone thinks Durant is skinny but he's 6'9 and weighs 230. Robinson is 7'1 and Robinson has more body fat% than Karl Malone who is 6'9 and weighs 265. I looked it up but I'm confused how could someone tell whose stronger than someone. Durant weighs more than Kobe,Wade,Ben Gordon,and T-mac but his arms are so much smaller. Robinson's arms are bigger than lebron's,ben wallace,artest and tim duncan but he weighs like 20 pounds less and is taller. Am I missing something.