I can't address international airspace, however in US airspace the controller is required to use the term "heavy" in radio conversations in the terminal airspace, but not en route airspace, with the exception of issuing traffic to another aircraft on a heavy.
( From the controllers handbook) Heavies are aircraft capable of takeoff weights more than 255,000 pounds whether or not they are operating at this weight during a particular phase of flight.
B757-300, B767, B747, B777,DC-10, MD
-11,Some DC-8s, Some B707s, A300,330,340, L-1011, C5, C141,C17,B-52, Some KC
-96.....a few others.
I don't understand everything I don't know about this.