How The Pecs Grow and Appear
Question: I've seen some pics of Arnold Schwarzenegger as a teenager, and sometimes his lower pecs look thicker than what they do in his later years. Is this a natural occurrence that takes place on the body?
It all depends on the body type, and depends on the bodybuilder.
Some people devote more attention to specific areas to take on their ideal appearance. As a result, they build that body part to their liking, and take on the shape they worked for. In this case, the chest.
The thing about bodybuilding is that YOU can decide how you want your body to look. It's almost like creating a masterpiece similar to a painter or a sculptor... You decide the final result.
During Arnold's career, he evolved as a bodybuilder, and in that process, his body took on different appearances and shapes based on the training and diet he chose.
Arnold did a lot of mass building, lower rep/heavy weight sets, or power lifting, in his teen years, then eventually began isolating the muscles more, to get that striated professional bodybuilder appearance.
Not everyone's chest will develop the same way. Some people may have to work a little harder, or lift a litter heavier... some may have to add a little more cardio or adjust their calories to show the definition more.
The basic idea here, is that the body changes based on your goal, and what you DO to get your body there.