It is normal for any QI (inductive charging) device to get warm while charging. And there is often a little bit of warmth even when it is fully charged and just sitting on the charger (since it is always topping it off some, and the device is still on and active).
Part of the warmth is the battery charging (and that will happen on all devices, wireless or not). And a small part is the device running. Part of the heat is the electronics in the charger. The rest is the imperfect transfer of energy from the charger to the device through the alternating magnetic field.