Bull, if you want to stretch the pec fully you'll note that your arm is horizontal, level with the shoulder and slightly behind moreso if you have great muscle strength.
For a fully contracted muscle the arm is now parralel to the torso and slightly pointing to the mid line.
I've always worked on the idea that the strongest part of any movement is where the muscle is fully contracted, where the lever cannot move any further, this is where resistance should be at the highest.
And if you want a really big strong muscle, an exercise copying the movement of stretched to fully contracted must be an exercise that you do regularly.
Based on that, if you look at the style of dip you commented on, there is more emphasis on the tricep, it may appear that your pecs receive the brunt of the work, but only in the stretch posisition, the weakest position for effective stimulus for growth.