Deep muscles are often not addressed well through most exercises, though doing otherwise will increase the size of any region they occupy.
For instance, stretching and working the brachialis can increase the size of the arms (hammer preacher curls), the soleus for the calves (heel raises with bent knees), and the pectoralis minor for the chest (dips or decline presses).
Here is another reason to try bench pressing with dumbbells, especially with the palms facing each other through a neutral grip that externally rotates the shoulder.
Subscapularis
The subscapularis is a deep triangular muscle, the largest and strongest of all the rotator cuff muscles. It can be felt below the chest right where it meets the front deltoid. It lies on the anterior surface of the scapula, mostly underneath the ribcage.
Growing the subscapularis will expand the chest, albeit non-visibly. This, along with pec minor development, likely also creates the illusion of a flat and square chest, layering density to the pec-delt tie-in area. This is considered an ideal proportion for this region within natural bodybuilding circles.
The subscapularis, especially the upper fibers possessing greater bulk, functions mostly to produce internal rotation foremost along with adduction, depression, and horizontal flexion/transverse adduction (pushing something in front of you).
The lower fibers mostly stabilize the shoulder during movements like shoulder abduction or extension.
The subscapularis therefore has nearly the reverse functions of the infraspinatus-teres minor complex. These muscles get overstretched and overloaded for optimal growth through a side lying rear raise. This involves horizontal extension/transverse abduction while internally rotated, so pulling something from in front of you backward, the opposite motion described here.
Dumbbell Bench Pressing
Dumbbells allow greater external rotation than possible with a barbell, by having the palms face each other, and more horizontal extension too. This elongates the subscapularis in addition to the chest and front deltoid. Make sure to flare the elbows to descend as low as possible.
Keep in mind that the subscapularis won’t perform quite as much work as through internal rotation, since the moment arm is shorter for horizontal flexion. This means the chest muscle will fail before the subscapularis on a dumbbell bench press. Nonetheless, it will still receive more growth stimulation than otherwise, reaching a stretched state along with performing some work at an optimal muscle length.
(As an aside, this concept also applies to the side lying lateral raise. It emphasizes the rear deltoid. The infraspinatus and teres minor are stimulated better nonetheless, but the lying side wiper exercise would provide more work.)
This extra stretch stimulates a unique form of muscle growth. Sarcomeres in-series are added, plus connective tissue via ECM and collagen to handle the overstretch. More sarcomeres allow higher active tension, which occurs when the muscles contract, at longer muscle lengths.
This encourages longitudinal muscle growth, a different source beyond just overloading the muscles with heavier weights at medium lengths. This more standard form of cross-sectional muscle growth happens on a barbell bench press.
This is similar to the value of decline presses or dips to address the pectoralis minor underneath the superficial pectoralis major, and this exercise also stretches and works the subscapularis, albeit not as intensely.
The pec minor fails to stretch enough on a regular bench press, requiring shoulder elevation as well. The subscapularis fails to stretch enough without some external rotation.
Both these muscles, the subscapularis and the pec minor, contribute to a larger chest measurement. They inflate the area even though they lie deeper than the lower and upper heads of the pec major.
This may also explain why fly movements were considered outer pec builders, even though we now know that growth generally occurs across the whole fiber: they grew the subscapularis.
Dumbbell Bench Presses for the Subscapularis
Muscle growth comes from different places including myofibrils, sarcoplasm, and connective tissue.
Overstretching a muscle leads to more connective tissue, while also adding sarcomeres in-series for myofibrillar hypertrophy. This differs from the growth of sarcomeres in-parallel that occurs from overload when muscles contract slowly through heavy weights.
Though controversial and not fully understood, sarcoplasmic hypertrophy, and energetic growth that includes glycogen and water storage, may increase more so with fatigue at higher training volumes. This associates with the pump and burning sensations via higher reps, shorter rest periods, and the positive phase of the repetition at shorter muscle lengths.
This creates metabolic stress, perhaps explaining partially why bodybuilders incorporate moderate-to-high rep ranges some of the time, unlike most strength and power athletes.
A sensible idea as a natural bodybuilder is to stimulate growth from all possible sources, hence why variety is a popular concept from experts in this domain.
Consider bench pressing with dumbbells, using a neutral grip, for the subscapularis. This allows for a unique form of growth due to overstretch. Overload this muscle through a movement with active contraction as well, such as a wide-grip pull-up for shoulder adduction, and you achieve complete subscapularis development for a larger, flatter chest.