To build chest mass, you need to break down the chest muscles through training. The process of repairing these tears leads to bigger, stronger muscles.
Most people turn to the bench press when they want to pack on muscle, but chest flyes are great for building mass when used correctly - form here is key.
If you're serious about building your chest, you'll need to train your chest at least twice a week and include a mix of compound and isolation exercises. You'll also need to get enough protein, and consider going on a bulk.