Casey and Arnold are probably the 2 most responsive BBers ever. The way they built muscle seemed effortless, i mean Arnold added almost 20" onto his chest in the first 4-5 year period of his training. He also had the biggest arms in europe, which could have been bigger still has he been able to build proportionate triceps to his amazing biceps.
As for Casey, yes he starved himself down and muscle memory helped him alot. But there are maybe a handful of people around who could have done exactly what he did and build the muscle mass he did. Check the pics of him when he was 18-19