I believe you are confusing pressure with volume or flow. Given equal pressure, you will get more volume of water out of the 3/4" pipe. (Area = Pi x radius squared) Do the math for the two pipes and you will see there is much more area in a 3/4" pipe than a 1/2" pipe. The only way to tell for sure what your pressure is is to use a pressure gauge. Pressure does not change with the diameter of a pipe. Over extreme distance you would have pressure loss because of friction, but it would be insignificant on the distance you are concerned with.