Our house is only 15 y.o. and is mostly/all original. On one side of the house the waterline is 5/8" (has 3 toilets, 2 showers & 4 sinks it goes to). The other side is 1/2" and a sink, frig, sprinkler system (likely added) and is plumbed for a full bath.

Our guess is the difference in the feeding lines is the amount of plumbing fixtures, but it could also be the distance from the line coming into the house.

I doubt this is a mystery to many of you, but if you could explain, we'd greatly appreciate it. TIA