Cribbed from the web, more complete answer:
The ICAO standard is for any aircraft capable of taking off at a gross weight of 300,000 pounds or more to use the term "heavy" in radio communications while below 18,000 feet above sea level. In the US, the threshold for "heavy" is 255,001 pounds or more. The term "heavy" may be omitted after initial contact with ATC.
The key word here is CAPABLE. For example, a Boeing 767-200ER with a maximum take - off weight of 395,000 pounds will always be referred to as "heavy" in air traffic communications, even if it is flying at less than 300,000 pounds.
The terms light, large, and heavy allow air traffic controllers to give the planes adequate clearance for wake turbulence, and to help the controllers assess the maneuverability of the aircraft, their landing speed, which runway they should land on, etc. It has nothing to do with the amount of fuel, passengers, flight phase, etc.
For example, if you hear an aircraft say "Tokyo Tower, this is United one two three heavy" the tower controller knows who the pilot is calling, what airline it is, what their flight number is, and that the plane has a maximum certified gross weight of at least 300,000 pounds. The controller then knows that United Flight 123 is not going to be able to make tight turns while on short final, and also to give other planes flying behind United 123 a little more time to allow the wake turbulence to dissipate.
The Boeing 757 has a maximum take off weight of 255,000 pounds so it fits in the "large" category in both US and ICAO definitions, but the highly efficient wing shape is thought to produce high wake turbulence for other aircraft so to improve safety they are also called "heavy."
