I did explain, reread my earlier post. The tris and delts are much more “active” in those exercises. The pectoral muscles insert at the humeral/acromial joint and are penniform in structure.
This means the fibres are arranged in a “feather-like” shape and are responsible for adduction of the humerus. Why do you think most powerlifters have small chests compared to their tris and delts? In comparison, bodybuilders however do not lack in this area because they do many exercises that take this into consideration, whether they know the physiology behind it or not. They perform many sets of cable flyes, pec dec and DB flyes. These are all examples of humeral adduction. Chad Waterbury even spoke of something similar to this in a past article I believe. I think he called one of the exercises “slide push-ups” or something like that and it basically entailed the principles I speak of in this post.