Muscle "density" is just a term given to someone who has built enough muscle to look solid. Many younger lifters don't have this look. They look soft, even if they are carrying some size. In reality, that look comes from TIME, being lean enough to see muscle seperation, and having enough size for the other factors to matter. No, you can not have that without building the size, working out for a long time and being defined enough to see the muscle you built.
"Density" is also used to describe that mature look that many bodybuilders in their 30's have. The muscles just look more solid as if they aren't going anywhere even if the guy started skipping meals. Vince Taylor has that look to him. Very rarely, younger bodybuilders will have it as well. Edgar Fletcher was one who always looked very dense.