With my new XS Max, I found the initial screen settings to be quite DIM: even after the common solution of turning off the Auto-Brightness feature (conveniently hidden in the Display Accommodations pane of the Accessibility settings), I had an issue with my brightness slider randomly failing to increase brightness beyond approximately the 70% brightness point. I'm a developer, but I was baffled. I spoke to an AppleCare tech, who suggested doing the "Reset Settings", which sort of helped, but then the issue came back.
I stumbled across a posting that described going into a dark room, turning on the Auto-Brightness feature, turning up the brightness slider all the way, then turning Auto-Brightness back off again. THIS APPEARS TO SOLVE THE PROBLEM: my XS Max screen is as bright as my iPhone 6s Plus!
I stumbled across a posting that described going into a dark room, turning on the Auto-Brightness feature, turning up the brightness slider all the way, then turning Auto-Brightness back off again. THIS APPEARS TO SOLVE THE PROBLEM: my XS Max screen is as bright as my iPhone 6s Plus!