Officer Kite - the regulations tell us that when the Max Takeoff Weight (MTOW), less the fuel necessary for a 15 minute flight (including takeoff, approach and landing at the departure airport) is more than the maximum go-around weight, a fuel jettisoning system must be available.
So its nothing to do with the size of the aircraft, it's to do with the aircraft's go-around performance at MTOW with an engine out.