Heat is a natural side effect of energy transfer, such as when charging. The faster the phone charges, the warmer it will get. And the phone charges more quickly when its battery level is low.
As long as you're using a quality USB-C charger (like the one that came with the phone), the phone and charger will communicate to work out the optimal safe charging rate. Even if the phone gets warm to the touch it shouldn't cause any problems - the phone itself will shut off to prevent it from getting hot enough to cause lasting damage.
If you're concerned about the heat, you can charge with an A-to-C cable attached to a standard USB charger; this will charge at a much lower rate resulting in lower heat build-up.