Actually water pressure does not change with the size of the piping...water flow does. If your pressure is too low, no matter the size of the piping it will not increase your pressure. Increasing the size of the piping will give you greater volume to the fixture involved. But if the flow rate from the service is cut down by flow restrictions such as calcium and mineral deposits, increasing the line size inside will do no good. First establish the pressure available from the city and the type of water service you have available. If its galvanized, you could be spending money on the inside that will only minimully help. The best way to tell is to take the house side of the meter off and determine the flow rate. By connecting a hose to the house side of the meter and turning on the water and reading the the maximum number of either cubic feet per minute or gallons per minute available, you can determine if you have enough volume for your fixtures. Each fixture has a fixture unit value which is equal to 7 and one half gallons per minute of flow. A kitchen sink 15 gallons per minute. The minum flow you would want coming into a house is about 15 gallons per minute. Any lower and no more than one fixture can be running at ta time.
Raymond Vin Zant