The hidden cost of oversizing becomes apparent during the life of the system.
With extremely few exceptions, every new hydronic heating system has at least one circulator. Some larger residential systems might even have a couple dozen. There’s a pump for every purpose; it’s just a matter of defining that purpose and picking the right pump.
All those who design hydronic systems use some method of pump selection. Some make good use of resources such as published pump curves and data to estimate the head loss of a given piping circuit. They find the operating flow rate where the head produced by the pump equals the head dissipated by the piping. They might even consider how upsizing the pipe size might allow them to reduce the size of the circulator.