The Air Tractor 802 is the worlds largest single engine aircraft. It carries almost the same load as the Firecat. Conair has two of these and perhaps more on order. (thanks to Yves Beaudet Abbotsford FSS for the information)
Specifications; Engine Pratt & Whitney PT6A-67AG (1,350 hp), wing span 58 feet, length 35' 7", Maximum take-off weight 16,000 lbs, empty weight 6,810 lbs., Maximum load 9,190 lbs