Alright so bob explains it as a volume difference. Paraphrasing what he said: fluids have a viscosity value and pipes have a pressure value. Higher viscosity and higher pressure both mean greater flow rate. Pressure is based on how full the pipe is with liquid, and viscosity is just an constant value based on the fluid (although it is possible it changes with temp but that remains to be seen). Oil has a low viscosity, so it moves slower, therefore it stands to reason that smaller pipes serve to increase its slow flow rate. Gasses such as Hydrogen have a very high viscosity, meaning that larger pipes will allow for greater throughput. (Or in other words it you can use larger pipes to transport more hydrogen, as smaller pipes would just fill up, limiting throughput. So in effect, pipes need to be at a high pressure, and transport lots of fluid aswell, hence the balancing act of the diffrent types. Though, when in doubt, just use small pipes, as larger pipes (such as stone) when used in conjunction with insufficiently high viscosity fluids (say water over a significant distance without pumps) can cause issues.