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There is no inside vs outside of the pec. Muscle fibers run the length of the muscle from the origin to the insertion. For the pec, this means from the upper arm to the breastbone. Therefore, you cannot contract one part more than the other. What you feel is the exteme range of motion that flyes bring into the movement. Your elbows travel further in a flye or pec deck than in a barbell bench press. However, this extreme stretch that many people look for is not good for you. your shoulder has a tremendous range of motion, but it sacrifices stability for it. If you push to the edge of flexibility, you sacrifice stability and risk injury.
If you do flyes or pec deck, do not overdo it. Your chest routine should generally be bench, incline bench, dumbell presses, standing cable presses, dips, etc. Notice that these are all compound joint movements. In real life, you do not move just one joint. How many joints does it take to kick a ball, swing a bat, punch someone? Use compound joint movements for the majority of your training.
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