ICAO and the FAA do have different definitions of "heavy" when it comes to callsigns. The FAA defines a "heavy" as an aircraft capable of takeoff weights greater than 255,000 pounds and ICAO uses 136,000 kg (about 300,000 lbs).
Now, for the 757: Although Boeing produces the 757-200 with an MTOG of 255,000 pounds, different airlines choose to limit their 757 MTOGs to different numbers (to reduce weight-based landing fees, etc).
UAL 757s, for example, have an MTOG of 240,000 lbs. You'll never hear one be called a heavy. Other airlines may designate Boeing's 255,000 lbs as their MTOG.
767-300's are always heavy in the U.S.