Like I said, the 4850 doesn't need as good cooling. The GPU is designed to handle a high amount of heat.
Also, they've programmed it to run the fan speed at a quite low setting (to reduce noise). If you increase the speed by about 7%, it can make it run significantly cooler.
9800GTX+ doesn't produce less heat (in terms of Watts output). It just has a bigger cooler (able to handle a higher heat output, which is needed for the 9800 GTX+).