Better structure. Reeves had really wide clavicles and a really narrow waist & hips which Arnold did not. Reeves had a 29" waist at around the same height as Arnold and his was 34" Reeves had better balance between the muscles a true Greek Ideal , Arms , neck and calves exactly all the same size. Reeves had better symmetry all of his muscles appeared the same shape Arnold's biceps alone were different sizes and shapes. Arnold was leaner and harder but chalk that up to 30 years of trial-and-error in nutrients and technology. Reeves stands alone
In this photo, no one looks as good. The man was The Man. And still is. Reeves physique was perfect. His neck, calves and arms were 18.5" and his legs 26" and his chest 52". This is the standard of symmetry that has long been abandoned to fulfill the fetid fetishes of size queen schmoes.
Arnold is overall the best drugs or not. But Reeves is Reeves.