You will notice most of the heat is at the beginning of a charge when the battery voltage is lowest. Needs to burn off the extra usb voltage.
The new version 7 ML-102 has a lot potential. It uses a switching li-ion charger chip, should produce much less heat and the 5v usb output boost converter could be capable of 2.1A. If the chips actually come close to what they claim it would make this v7 really great.