The reason for the heavy callsign is not for fuel, but for wake turbulence separation.
Heavys have to have 5 miles cleared behind them, 757-200's 4 miles, and regular airliners 3.
In the US a heavy qualifies for an airplane that has an MGTOW of greater tahn 255,000 pounds. Note that it does not have to actually weigh that...just be able to weigh that.
Chicks dig winglets.