For Size, Cable Flyes Beat Dumbbell Flyes
It's important to remember which direction the chest fibers run. They don't travel vertically – they're much more horizontal in path. So we need to use resistance that directly opposes the muscles' fibrous direction.
Using dumbbells during chest flyes does this, but incompletely; they only zero in on the pecs for about half of the range of motion. On the top half of the lift, the force angle is directed downward, thanks to gravity, and the chest muscles really have little to no involvement in holding the weight up.
Compare this to setting up between two low-cable pulleys for a chest flye. It creates a world of difference.
Cables create constant tension by way of their outward force, directly opposing the path of the chest fibers and requiring a much stronger contraction for literally 100 percent of the lift.
Instead of fumbling with 50-pound dumbbells for flyes that turn into bench presses by the end of the set (be honest!), really torch your chest by using cables instead. It'll feel better on your shoulders too.