I've got an entry level Onkyo(505) that does the same thing, though I haven't measured it, so apparently Onkyo has designed their receivers to run hotter. This seems like cutting corners to get a product to market cheaper to me. Apparently their OSD is also pretty lame in some models and they scrimp on things like speaker terminals and amount of some others like toslink or composites. But I guess if you don't need them...
Have Onkyos always run hotter or just in the last couple years? My Denon 3803 barely gets moderately warm, and I've had it for about 5 years. My onkyo starts sweating with the volume set at zero!