Oh and I forgot:

"so the only way to gain mass in the middle of the chest doing chest workouts?"

Keep in mind that no muscle group can ever reach its full potential for growth if you're not doing your major compound lifts - obviously you're doing bench but squats and deadlifts are ESSENTIAL to really add mass (you only listed your chest workout so I wasn't sure). If you're only lifting your chest because that's what you want to improve the look of, not only will you look weird but you won't be as big or as powerful as you can be.