My house has six rooms, but the 3400-watt heat pump is only enough to heat two of them. If it costs tens of thousands of dollars in the US, say US$25000, you would expect the resulting installation to be able to heat or cool about 200 rooms rather than 2, producing 340 kilowatts of heat output (1.2 million BTU per hour) and consuming 94 kilowatts of electrical power (430 amps at 220 volts). Indeed, because houses gain and lose heat only through their surfaces, you'd expect the 100× bigger US$25000 heat pump installation to be able to heat or cool a 2000-room building rather than merely 200.

Most houses in the US have less than 20 rooms, let alone 200 or 2000, so it's not mostly because houses are bigger.