Why does my iphone get HOT while charging using the car cigarette lighter?

When charging using normal usb to pc or wall outlet, the phone doesn't get hot. When charging using a usb to car cigarette charger (12V dc to 5V/1A) … The phone gets pretty hot.

Why does this happen? Is it bad?

It depends if you're using it WHILE it's charging. Sometimes, I charge my iPod with a speaker dock, and it gets really hot if I use it for even a minute. But if you don't use it while it's charging, then I don't know. Hope this helped!

I don't think so, it charges faster in a car. I do it all the time and never had an issue.