Your pec, like any muscle, can only grow along the fiber. Pec fibers run from the point under your front delt/attached to humerus to your sternum and clavicle. That means we can only hope to grow, essentially, areas top to bottom, not inside to outside, along the path of the fibers.
Muscle fibers are like rubber bands. Put a rubber band between two fingers and stretch it; the tension is the same everywhere in the fiber unless you pin one part of it down (which we can’t do with muscle fiber).
So there is really no growing “outter chest.” I assume you’re referring to the area near your delt when you say outter, so if that areas lacking, build up with movements that target that level of fibers. Think any incline, really any angle at all all the way from 15 deg to just shy of 90. Find what works for you. Any movement that brings your arm/hand above the middle of your chest (and as a matter of fact in-line with the fibers you want to grow).
If you’re referring to the “lower outter” that’s a matter of doing the opposite, and targeting movements that align the path with the lower fibers.
photo attached for reference. This targeting of fibers is more biasing, in that you can’t truly shut off parts of your pec. Certain movements just generate more tension on certain fibers based on angle and execution.
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