Nibal,
It is all about controlling your pressure and flow. Keeping in mind that you simply need to stay within the available pressure and flow limits. So, we know that if the system is trying to size your pipes bigger, it must mean that you are getting too much friction loss through the give sizes of pipes based on your flow and velocity set. If you are set on keeping the smaller sizes of pipes, you need to play with the pressure variation factor and velocity when sizing your lines. If it is still not working for you, you need to make sure the flows you have on a given valve will be handled through your desired size of pipe (without breeching the velocity that you have determined).