Meters measure how many gallons of water flow thru it. Nothing to do
with pressure. The main difference between using different size lines
is GPM's (gallons per minute). A line can only put out what is coming
into it, not more. But, going down to a smaller line will reduce
GPM's. Pressures may vary depending on length of run, elevation and
other appliances in the line.
Pipe design is for a reasonable pressure drop in the pipe at a design
water use rate. I would expect 1" pipe was used because there is a lot
of pipe in the run to the barn. A 1/2" pipe to the barn would have a lot
more pressure drop (which would reduce the max flow rate). A 5' length
of 1/2" pipe in the barn would have the same pressure drop per foot, but
that would not be much.
(The equivalent in the electrical field would be increasing the wire
size in a run to the barn to reduce the voltage drop.)
A 3/4" meter would have more pressure drop across it than a 1" meter (at
the same flow rate). From what you describe, it shouldn't be significant.
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