Suppose we have a tank containing compressed air. If a hole is made in the tank the air will start flowing out. The mass flow rate of exiting air will be heavily dependent on the diameter of the orifice (ie the narrowest opening on the system) - the bigger the orifice the greater the initial mass flow and vice versa.
Now, let's say we want a certain mass flow rate (x) coming out of the tank, whilst maintaining a constant pressure in the exiting gas. We can achieve the desired mass flow rate by making an appropriately sized opening in the tank (I'll call this opening O[i]) and adjusting the pressure using a pressure regulator. The issue here is that the pressure regulator could have a smaller opening (O[pr]) on its inside, ie O[i]>O[pr]. This would consequently reduce the mass flow rate coming from the tank.
It's obvious that a pressure regulator can't be used above a specific desired mass flow rate. What other devices could be used? A wider pressure regulator maybe?
I have included a diagram of a pressure regulator, which clearly shows the small opening inside.