Why does my phone get so hot

This is a belated answer so hopefully it can be of use to folks who search for and find this question.

It is normal for the phone to get warm/hot depending on how much work it is performing. Downloading/watching videos consumes more power than texting. Games consume a lot of energy as well. Lowering the screen brightness is one way to lower the power consumption and that will have an impact on the heat generated.

As for the phone being too hot, I guess that's subjective. The iPhone has multiple temperature sensors on the logic board. If the phone truly does become too hot for component safety reasons, then it will shut down automatically.