Gas main sizing

I don’t know about engineering principals but in Gas Terms the logic is simple.
The gas mains in the street run at a much higher operating pressure than do households so the pipelines can be of a smaller diameter feeding up to the gas meter at the property. There is a pressure regulating device fitted to the gas meter than reduces the mains gas line pressure to that which is required to be delivered to the householder and as a consequence the pipework after the meter has to be of a larger diameter to allow for any pressure drops and carry the appropriate volume of gas required to supply the appliances installed.
The gas Utility also applies a “fixed factor” for billing purposes and as the pressure supplied after the meter increases so does the factor (supercompressability). Hope this helps.