https://arstechnica.com/gadgets/2023/10/apple-and-devs-plan-software-fixes-for-iphone-15-pro-overheating-issues/

Some owners of the new iPhone 15 Pro and Pro Max have reported over the last week and a half that they've noticed their phones running a bit warmer than older iPhones. We observed that the phones could run hot while working on our review, "sometimes to the point that they were uncomfortable to hold without a case," and other reviewers and users have noticed the issue as well.

The problem is real and serious enough to have merited an official response from Apple over the weekend, shared with MacRumors and other outlets. Apple says the main culprit at first is "increased background activity" that occurs as users set up their new phones for the first time (which I've also noticed in other iPhones and iPads and is pretty typical).