Kettlebells work better than dumbbells for flyes. Because of the position of the resistance, the pecs are under load for a greater proportion of the range of motion.
With dumbbells, the muscle tension on the pecs is greatly reduced at about 60% of the range of motion. This can make the reps less effective, especially when you're using a method where you must maintain constant tension to create an occlusion effect.
Didn't it seem that the bodybuilders of yesteryear had bigger chests? (We're talking chest circumference here and not pec size, mind you.) Ellington thinks so and he knows why - it was all due to the all-but-lost art of ribcage expansion.