Smaller pipes do not = lower pressure. The pressure remains the same with small or larger pipes. The volume of water changes with sizes of pipes. Get a pressure guage that attaches to a hose valve and check it near the house, preferably after the pressure regulator. You stated watering you lawn is a problem also. Does your water line split after the meter and go to the house and to your sprinkler valves? If so, is the pressure the same to both? Does your meter at the street sit at a lower altitude than the house by say 20+ feet where your home is higher? This can reduce pressure also as water is pushed uphill. If your water pressure at the street meter is around 35 psi then you will have some of these problems.
I'm not a plumber. I'm a dentist by trade. I do work alot on my own home and live on top of a hill. I had this same problem when I moved in 20 years ago and did several things. I had old pipes so i replaced the corroded galvanized pipes from 1948 with copper. This helped with volume of water but I still had only 30 psi at the meter. The water company then installed a pump in their system to make the pressure closer to 70 psi at my meter which helped immensly.
I recently placed a booster pump at my mother-in-laws home along with a pressurized holding tank. She also had about 35 psi at the street and the water company was going to do nothing to increase that. Her home was about 30 feet above the meter. Pressure at the house was about 28psi.
I placed a water pump after the meter that pumps the water into a pressurized holding tank. There is an internal bladder that fills with water with air surrounding it in the tank. The pump's regulator valve detects a low water pressure, turns on and pumps the pressure of the water up to be held in the tank. You will need a valve to prevent back flow to the meter. The tank is used to keep the booster pump from cycling and burning out the contacts. I was able to increase her pressure to 75psi and in turn, helped with her sprinkler system also.
Check for the easy things first. I added all this as a possible fix. The best thing to do is to get 70 psi or so to the water meter then you just have to make sure your pipes and valves are clear. You could also have too many sprinkler heads on one sprinkler valve.
I hope this has been of some help.
Jim one-eye 