/3/ - 3DCG

So pectoral muscles aren't just bumps that grow off your rib cage. they're fibers that connect to you sternum and then to your shoulders. If you want big intimidating muscles you've gotta show the bulging irregular cords and how they connect. For really exaggerated muscles you can show these creases in the skin the theoretically would form from the tendons that connect the muscle to the bone being over grown.

The easiest way to make these in 3D would be to block them out in your 3d package and then add the details in zbrush as a normal map. IF you dont want to sculpt then make the topology of the muscles follow the tendons and then have the edge loops alternate in height.
Remember if it looks wrong then change it until it looks right.