Muscle maturity is usually associated with years of training that particular muscle to the point where its very dense. The muscle has more separation and more muscle fibers in a certain space. You just have to look at a picture of Vince Taylor to know whatt I mean.
Chris V is right...... it comes with years of dedication
but you can even see it in the pros..... look at pictures of the older guys....... ronnie coleman, kevin levrone, shawn ray, flex wheller, nasser el sonbaty, vince taylor compared to the young pros........ king kamali, dennis james, melvin anthony, craig titus
its not size, its the details that comes after years of having the size