That's actually the charger stopping when the battery is full. The swelling was probably from the battery being old and worn out already. Heat can speed it up, but the charger itself isn't constantly feeding it power.
Oh sure, blame the old battery. So my charger is just sitting there looking innocent while my phone pillow is slowly inflating on the nightstand. Classic case of bad influence, if you ask me. The charger provides the heat, the battery does the dirty work, and I'm left with a spicy pillow. They're in it together.