There are four classes of aircraft weights:
A. Super: The A380 is athe only aircraft to fall into this class as of right now.
B. Heavy: Aircraft capable of takeoff weights of more than 300,000 pounds
C. Large: Aircraft capable of takeoff weights of more than 41,000 pounds, but less then 300,000 pounds
D. Small: Aircraft capable of 41,000 pounds or less
maximum certificated takeoff weight.
In the U.S., this just recently changed, with the reclassification of the Boeing 757 as a "Large" aircraft."Revised is made to harmonize FAA weight category standards with those of the International Civil Aviation Organization (ICAO). All aircraft with a maximum certificated takeoff weight of more than 41,000 pounds but less than 300,000 pounds maximum certificated takeoff weight will now be classified as a “Large” aircraft according to FAA standards. Aircraft with a maximum certificated takeoff weight of 300,000 pounds or more will now be classified as a “Heavy” aircraft according to FAA and ICAO weight classification standards.
This change reclassifies all B757 aircraft as “Large” aircraft; however, controllers are required to apply the special wake turbulence separation criteria as specified in paragraph 5-5-4. This change cancels and incorporates N JO 7110.525, Appendix A, Aircraft Information Fixed-Wing Aircraft, effective April 8, 2010."
Aircraft are classed by the total amount of weight they are certified to takeoff with, even if the are not operating at that weight during any phase of the flight. Basically, an empty Boeing 747 is called a heavy just the same as a full one, for example.
Hope that helps!