I believe the true "heavy" designation is not solely based on weight or runway usage. Specifically, it has to do with the wake that the aircraft creates in the air. Using the heavy designation reminds everyone that extra spacing and caution is needed around these aircraft.
While the "heavy" term applies to big and loaded aircraft, it also applies, for example, to the 757. This aircraft still gets the "heavy" designation because of the unusually large wake it leaves in its path.