If your goal is chest size and your pecs are being stubborn, add a band (a form of accommodating resistance) to your flyes to increase the metabolic stress.
Banded Dumbbell Flyes
Banded Kettlebell Flyes
Grab a light to moderate resistance band and wrap it around your back, preferably the lats.
Grab the dumbbells or kettlebells with the bands around your thumbs or whatever grip is comfortable for you.
Lie down on a flat bench holding the dumbbells or kettlebells at your shoulders, palms facing inwards. Bring the dumbbells up until your arms are almost fully extended.
Slightly bend your arms and arc the weights down to your sides until you feel a stretch across your chest. (Don't bring the dumbbells to your ears because you'll place too much stress on the shoulder joints. Kettlebells allow more range of motion though.)
Squeeze your pecs to return the weights to the start position.