The sizing of a septic system is determined by the soil in the ground. Sandy soil with low silt content is best. If you have clay soil, a high water table, or a water table that is elevated at certain times of the year, then importing sand and/or raising your system is what has to be done to ensure that your system will work properly.
The sizing of a septic system is mostly based on the building it is servicing. It is not based on who it services, rather who it could potentially service, to reduce the need for upgrading septic systems. The system must meet the maximum possible daily sewage flow the building could produce.
Septic systems require certain clearance from wells, swimming pools, property lines, buildings, rivers, lakes, ponds, reservoirs and springs. Therefore they are not necessarily placed where they are aesthetically preferred, but where the system can meet all of these requirements.