I agree with mexcheme.
Control volume means a device is used to control the volume; commonly valve is used.
Closed system is used usually in refrigerant system.
Do you mean that smaller diameter increase the pressure drop? I think to increase the pressure (reducing pressure drop) a reducer is needed, in other words using smaller design is better.
For your question, in my opinion, regulator is used to overcome the pressure drop and to increase the flow...