Why does my Android device generate more heat on an AC charger?
I'm not an electrical engineer, but I'm pretty sure there's either


more current flowing through a wall charger than there would be a USB charger or...
the same amount of current which is just flowing faster from the wall than from the USB port


which is why the wall charger will fully charge your battery faster.  That heat is totally normal and nothing to worry about (unless it gets hot enough to burn skin, then I'd recommend not touching it).