Two iPhone 7 Plus units I have run warm even when doing nothing--sitting with blank screen. And this is after 4 weeks of ownership, not new. Anyone else?
My 7+ and my wife's 7+ seem to be about the same as my old 6+ and her old 6s.
I'd be checking out battery usage and location services. That should get you going in the right direction to figure out what's causing your phone to run hot.
If you iPhone is warm, even when not being used, something is going on in the background. You battery HAS to be taking a hit, too, as heat is only a biproduct of what is going on.