The trade is full of warnings about the perils of undersizing or oversizing a heat pump. Symptoms vary from having a cold house to reducing life span from frequent stop-starts. In my mind, the only time a heat pump is the correct size is when it is supplying full power on the coldest day to give adequate warmth. At any other time, it will be running at reduced power… in other words, oversized. At the low power end, the pump will be just ticking over at the start or end of the heating season