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Faster or more charging requires more power and therefore more heat is produced. If your phone is at 20% and you charge it, it will be charged at a higher rate than if you were charging it at 90%.

If you have an inductive charger, a wireless charger, those heat up due to the nature of their design when in use.

Edit: Don't rule out malfunction as the cause. The charger, cable, or phone could be creating an unsafe operating state.

You should post your phone, the cable and the charger (specs) Also the soc at the beginning of the charging process.