To me the pcb layout and length appears to be a twin transformer setup. That explains the low amount of heat produced as neither transformer has to run at a high relitive load.
The lower output models could be a single transformer design which may lead to more heat. You simply need to read reviews and check the specs.
By the way this is not a definitive gauge of how loud the psu is under load. Under a dual 8800gtx setup for instance there would be a higher load than the 320w loadtester w1zz uses.