If your heat pump is too big, it will cycle on and off more frequently than a properly sized unit. This can lead to increased wear and tear on the unit and may shorten its lifespan. A heat pump that is too small will have to run for longer periods of time in order to achieve the desired temperature. This can also lead to increased wear and tear and may cause the unit to overheat. If you are unsure of the size of heat pump you need, it is best to consult with a professional installer. They will be able to help you choose a unit that is properly sized for your home.