If you have spent any time looking through human anatomy books and illustrations, particularly for muscle groups, because we like getting those muscle to be well defined, well developed and occasionally unreal in size and definition, you may have noticed a weird muscle group under the Pectoralis Major, the pecs we often wish to be well sized and powerful looking. The most "heroic" muscles of any humanoid body, The Pectoralis Major project a air of strength and toughness even the most insane biceps and triceps simply do not. The Pectoralis Major, as contraction muscles, are most responsible for rotating your arms inward, like hugging, a hook punch, reaching forward.

But the Pectoralis Minor is responsible for the boarder rotational movements of the shoulder, usually those associated with the scapula, or shoulder blade. This muscle sees enormous development in winged creatures like birds dragons and in fact function as the main muscles responsible for flapping wings. This would help explain how wings on Dragons whose wings are separate and distinct limbs coming off the shoulders work. We will have to work on "butt wings" later. But I suspect it will have a lot to do with the Gluteus Maximus.

The Pectoralis Minors, in humans and most humanoid anatomies, are small, comparatively UN-developed, seeming utterly vestigial compared to the Pectoralis Major, Deltoids, Trapezius and other muscles associated with arm and shoulder movement and control. They are still able to provide some respiratory action, breathing in the upper lungs, and rotating the scapula, allowed more movement forward.

Because the of nature of the movements the Pectoralis Minor enables it is extremely hard to develop these muscles and make any notable gains in mass or strength. But what if you could build these muscles with any degree of ready effect as the Pectoralis major? To start, I think it would make for very powerful breathing as you would have larger stronger muscles allowing for more inhalation to the point generating a noticeable breeze. This would also enable and louder stronger voice, greater endurance, and a much larger upper body proportions. This mainly in the chest.

A large Pectorialis Minor muscle would need more room, they would easily push out the Pectoralis Majors. The concept of "moobs", male boobs, which is normally males who develop fatty, lactating breasts over often muscular "pecs", through frequently depicted as just largely, rounder, less well defined pecs. Though these not "sacks of fat" as female breasts are normally depicted. This would seem more a female bodybuilders ideal compromise between having huge pecs and still have breasts, boobs, uhm... anyway, Not only more and larger chest muscles, but a broader chest and the lungs would be encouraged to increase their volume to hold more air. So a broader chest overall and more of a justification for "super pecs", Pecs of inordinate size even to rival that person's own gluteus maximus. Having a chest literally as or more muscular than your own ass. The Glutes being typically the single largest muscle groups in a humanoid body. This owing from using legs as the primary mode of locomotion, instead of arms for swimming or flying.

In function, larger stronger Pectoralis Minors would mean, in practice, being able to swim more effectively, flapping ones arms for flying (given appropriate body structuring and/or equipment), the increased reach due to the broadening of the chest would result more reach. The strengthening of this often overlooked muscle group would also enable other muscles to grow more potent as well as they would have to balance against or complement the "Pecs Minor". Those would also enable more movement options or just make them easier.

The "Pecs Minor" is an atypical muscle group with some awesome ramifications if developed like other muscles. Knowing it is there and what it generally does means, like the more overt muscles, like Pecs Major, Biceps, Triceps, Delts, Traps, Quads, Hams, Abs, and Glutes, a dedicated bodybuilder or strength builder would be foolish not to work on these as well.