[quercusa]

All aircraft produce wingtip vortices that create wake turbulence in flight. The vortex strength increases when the aircraft is heavier and when it flies slowly. Thus, the term "heavy" (unlike light, medium and large) is included by heavy-class aircraft in radio transmissions around airports during take-off and landing, incorporated into the call sign, to warn other aircraft that they should leave additional separation to avoid this wake turbulence.

https://en.wikipedia.org/wiki/Heavy_(aeronautics)