When sizing pipes for natural gas or propane gas services several things affect the size required. Total BTU's that would be used if everything was on at the same time is one. Iron or other rigid schedule 40 pipe, soft copper type L, or whatever else may be used. And the distance of the pipe runs.
Here are a couple sites with info and charts
.... propane gas pipe sizing
http://www.engineeringtoolbox.com/propane-gas-pipe-sizing-d_827.html (http://www.engineeringtoolbox.com/propane-gas-pipe-sizing-d_827.html)
.... and one on natural gas pipe sizing
http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html (http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html)