Overheating brand new iPhone 13 Pro Max

It's not overheating unless it displays a screen telling you it needs to shut down because it's overheating.

It is completely normal for it to get warm or even hot when charging or when playing a graphics and/or processor-intensive game.

Do you have a case? If so, that could be part of the issue.

There are a variety of reasons:

• Are you both using cases and the same cases?
• Are you both using the exact same apps?
• Do you have anything else that is using power as seen as using battery in the Battery settings?
• When you say his phone doesn't heat, did you ask him or did you feel his phone after using the same charger for the same amount of time?