World Records in any type of aviation, including modeling, are usually attained under the strict guidlines of the FAI (Federation Aeronautique Internationale)
from the FAI website ;
FAI is the world air sports federation, founded in 1905. It is a non-governmental and non-profit making international organisation with the basic aim of furthering aeronautical and astronautical activities worldwide.
Besides developing aeronautics, one of the activities of FAI is to certify and register records. Some of the first records in aviation were registered as far back as 1906 by FAI. Some of the greatest feats in aeronautics and astronautics have been put in the books of FAI. All disciplines are involved, from Spacecraft to Human-Powered flight.
Under FAI rules any model competing for a "world record" can weigh no more than 5 kilos (11.023 lbs) full of fuel.
http://www.fai.org/records
Edit : Eddie may be on to something with the UAV angle. FAI does have a class for UAVs with heavier weights. See
http://www.fai.org/uav/