Our irrigation guy said our builder's plumber installed a 1 1/4" pipe from the water meter to our house. He said he had never seen anything but a 3/4" or 1" pipe run from the water meter in his 30 years of irrigation. He said this has caused our home to have low water pressure. Our irrigation sprinklers are weak - more like a stream of water than a spray of water. Our house only has about 42 psi of water pressure measured at the outdoor spigot. I read that 60 - 70 psi is optimal and 45 - 80 psi is considered in the "normal" range. So, I guess we are below normal. I've already taken out all of the flow restrictors from my shower heads and I can only get about 2.0 gpm from shower heads and a little less than 4.0 gpm from tub faucets. Do I have a problem I need to ask our builder to fix? We are still in the 1 year warranty period.
Two different concepts here - pressure and flow rate.
For the same size pipe, you can increase the pressure to get a higher flow rate because the higher pressure "pushes harder" to get more water through the pipe.
For the same pressure, you can increase the size of the pipe to get a higher flow rate because the larger pipe is less restrictive.
Your body needs some amount of blood circulation to live. If you don't exercise and your blood vessels are narrow, or you eat poorly and they're full of cholesterol, then your blood pressure must go up to keep the blood flowing through your body at the appropriate rate to live.
Here's a link from Oklahoma State University that mentions 30 psi as the ideal pressure at a sprinkler head, but also mentions ~15 psi pressure drop through all of the components in a typical system.
That said, if your pressure is 42 psi, and there's a 15 psi drop, then you should be at 27 psi at the sprinkler head. It's 10% low but I don't think that should be so much to make the sprinklers weak. What's more, from the troubleshooting section in the document I linked,
It is possible that the system was installed with undersized pipe and too many sprinkler heads on a zone. Low pressure can be addressed by changing to lower flow nozzles or decreasing the number of heads in the zone. Contact an irrigation contractor for assistance in evaluating your system.
The other steps are looking for leaks in the system and making sure all the valves in the system are fully open. More excerpts from the document:
Residential water pressure is ideally about 40 to 60 psi
and, regarding ideal operating pressures for sprinkler heads,
For rotors, this is about 45 psi, with an operating range of 25 to 65 psi
For spray heads, it is 30 psi, with an operating range of 15 to 30 psi
If you've got 27 psi at the sprinkler head then you should be within the operating range. The 27 psi estimate is also assuming the 15 psi pressure drop in the system. The irrigation contractor has the ability to lower the pressure drop between the faucet and the sprinkler by... wait for it lol... using larger-diameter piping and/or by using fewer sprinkler heads in each zone.
The fact that your contractor is saying a larger-diameter pipe is the PROBLEM is highlighting their ignorance about how fluid systems work and I'd highly recommend you find a different contractor to come in and offer a diagnosis for what's wrong.
The irrigation contractor should have measured the water pressure FIRST to determine an appropriate piping size and number of heads per zone and then worked from there. Unfortunately the remedy here is probably to have the piping removed and replaced. It may be possible to split existing zones into multiple sub-zones and leave most of the piping in place, but again I'd highly recommend you find some other contractor come in and give their advice on how to proceed.