The 757-200 is not "heavy" at 255,500, but the 757-300 is "heavy" at 260,000. The -200 is 500 pounds over the "heavy" requirement, so why not say "heavy"?
You say "heavy" in your callsign no matter what you weigh. It's what your aircraft is capable of, not what it weighs. ICAO doesn't use "heavy" in call sign. Their controllers are smart enough to figure out aircraft separation by themselves.