The delts are more like the endurance muscles you see with calves or forearms and they can handle more reps. Some athletes who are really gifted can do heavy barbell or dumbbell presses and just grow easily. If you have problems gaining size try giant sets (where you combine 4-5 exercises and complete them consecutively with no rest between). It is really intense and you will feel a killer pump on your shoulders. Do a giant set with dumbbell lateral raises, dumbbell shoulder presses, upright rows and bent-over lateral raises for 10 reps each exercise, or 40 reps to total. If your shoulders are not responding from heavy training, this type of advanced supersetting will really work.