Twice the LEDs: That means more heat coming from more LEDs. This is especially a problem at full brightness.
2.5X the power needed: The brightness efficiency is lower because the new iPad has more pixels (which means more transistors) compared to the iPad 2. More pixels and transistors take up more space, meaning less opportunity for light to pass. “So they basically have to blast light through the LCD to make it come out.” Soneira adds: “I measured the LED power at maximum brightness–it’s two and a half times greater than on the iPad 2.”
Battery generates more juice: The battery has to push out more power. This makes the battery warmer.
Traditional LCD technology: Sharp’s power-efficient IGZO technology was not ready for the new iPad. That forced Apple to use traditional–and less power efficient–amorphous silicon tech.[To be fair the older iPads also used this tech but perhaps Apple was hoping to go 100% IGZO to offset the above].
The biggest heater in the new iPad is the new processor which has four graphics cores. If you look at the heat maps that Consumer Reports and Tweakers did, the center of the heat is right where that A5X sits on the left side of the device.
As a bonus, don’t forget those hot and schweaty Qualcomm LTE chips which bring that “faster than home broadband” goodness directly to your 4G iPads.
With all of the above, it is a minor miracle that Apple managed to keep the temperature to within 10-15 degrees of the previous versions.