What you're saying makes sense. However, I think the capacity requirements for water/sewer are determined by law, much like the hard requirement that habitable

What you're saying makes sense. However, I think the capacity requirements for water/sewer are determined by law, much like the hard requirement that habitable residential spaces must have egress windows. In the USA there are similar laws around septic systems for detached houses, where the capacity/size of the septic system is determined by the number of bedrooms in the house.