The Post Quartermaster wrote:
I'm going to run a water line out to a little outbuilding I have out
near the barn. The line I am going to tap off of is probably a 3/4"
line. If I run a larger diameter, say a 1 or 11/4 inch from there to
the outbuilding would that result in any increase in pressure at the
outbuilding or would the fact that there was a 3/4" feed negate any
increase?

The larger diameter will reduce the additional losses added by the longer pipe,
compared to the same added length of smaller diameter pipe. The larger pipe will
also mean that a large valve at the end of the added pipe can cause a larger
pressure drop to be seen inside the house where the new pipe attaches compared
to a smaller pipe.