According to the Australian Chicken Meat Federation (ACMF), a myth has been busted by students from James Ruse Agricultural High School in New South Wales.  The myth is that Australian chickens grow bigger due to breeding and "not hormones, antibiotics or genetic modification"

The story via WorldPoultry.net reports that the school sourced and hand-raised 15 egg-laying chickens and 15 meat chickens as day-old-chicks from a commercial supplier.
All chickens were fed the same standard chicken feed product, made mostly from cereal grains and protein sources, obtained from a local feed supplier.
After six weeks, the average weight of the chickens bred for egg laying was 592 g while the average weight of the chickens bred for meat was 2388 g.

The conclusions are probably right, and breeding is a huge component in increasing size, but when you have 80% of Australians believing that something such as growth hormones are added to chicken I’m not sure it’s best to look to school kids to assure the population.  I especially don’t think you want to use statements like  "The truth is far less dramatic as was proven by this recent school project." as Dr. Andreas Dubs, executive director of ACMF was quoted as saying.