HVAC Guys' Q: Heat Pump Design Secret

I’m trying to recall something from an HVAC training course I once took. It had to do with the fan coil inside the air handler for a heat pump. I believe we learned the coil is sized properly for A/C only. As such, it will always be undersized for heating hence yet another reason why supplemental heating coils are needed. Am I remembering correctly? - Jinx