Three of the biggest sources of heat within a PS3 are the RSX graphics processor, the Cell processor, and the power supply. Many of the original fat consoles came with power supplies with the model number of ZSSR5391A. Those power supplies are only 66.5% efficient, meaning that lots of unnecessary heat is generated by the power supply.
I’m lucky, my PS3 has the much more efficient APS-226 power supply, which is 83.13% efficient. That’s a lot better than the other model, but is still contributing to the amount of heat that the console has to exhaust.