Ohh know not this question again (Check this link to see how long this argument lasted - http://airliners.net/discussions/tech_ops/read.main/11570/)
The B757 is classified as a heavy for ATC separation for wake turbulence purposes. Some people are confusing this term used for ATC separation purposes as the definition of a heavy aircraft.
Below is an excerpt from a breakdown of an ICAO International flightplan:
Item 9: Aircraft type and wake turbulence category/number
Wake turbulence category— Indicate wake turbulence intensity created by
the aircraft :
H — Heavy (aircraft with a maximum certificated takeoff weight of
136,000 kg/300,000 lb or more).
M — Medium (aircraft with a maximum certificated takeoff weight of
less than 136,000 kg/300,000 lb, but more than 7,000 kg/15,500 lb).
L—Light (aircraft with a maximum certificated takeoff weight of 7,000
kg/15,500 lb or less).
The B757 is has a *MTOW of 255,000 lbs, so obviously it falls short of being a heavy. But because of the wake turbulence that it creates (or so they say), the B757 CAN BE CONSIDERED the equivalent to a heavy at times.