"density" refers to appearance in this case, not actual physical structure. the muscle just looks harder, denser. dorians "grainyness" helped to make him look more "dense", coupled with his very low bodyfat and subq water stores. ronnie was shredded, and round.. didnt look quite as dense or grainy because his muscles looked softer and more baloon like, except for his arms.
ronnie is the better bodybuilder but the term density is real.