The design of relief valves involves complex calculations that consider many conditions such as fluid properties, inlet piping, outlet piping, and operating conditions such as temperature and pressure. It may also involve testing to ensure that the design capacity is actually achieved. You can find a good document describing relief valve design from Crosby Valve (and other manufacturers probably provide similar information): http://www.isibang.ac.in/~library/onlinerz/resources/enghandbook3.pdf

For a compressible fluid, it makes sense from an understanding of engineering flow calculations that the outlet of a relief valve would be larger than the inlet. Downstream of the relief valve, the pressure is lower, often much lower, so the compressible fluid will expand. You need a larger pipe for flow of the same mass of material at a lower pressure, the material will occupy a larger volume.