ndi's theory is very good -- I would also place my bets on the "charger" being a 115/230V. His theory would work if all the charger consists of is a 1:10 transformer, a rectifier (with capacitor), and then this LM7805. At 230V, we'd be feeding 23V to the LM7805; and at 115V, we'd be feeding 11.5V to the LM7805.

The thermal dissipation of one of these devices is 65 Cel./Watt (junction to air). While the designers of this charger believe it was sufficient for older model phones, we're talking about a smartphone with significantly higher power demands pushing this chip to its maximum current output.

Nowadays, chargers are probably made with a buck converter (takes higher DC voltages and steps it down) in light of new efficiency requirements.