Why heavy?

[DBE]

Ok: here you go..

The term "heavy" is in relation to the amount of wake turbulence (i.e. vortices) generated by an aircraft.

The strongest vortices are produced by heavy aircraft, flying slowly, in a clean configuration. For example, a large or heavy aircraft that must
reduce its speed to 250 knots below 10,000 feet and is flying in a clean configuration while descending, produces very strong wake.

So, in order for other aircraft to be warned about eventual turbelences generated by such giants, the "heavy" term is added to their callsign.

Hope this help!