iPhone XS has slightly smaller battery and more RAM than iPhone X
https://www.theverge.com/circuitbreaker/2018/9/19/17879438/iphone-xs-max-xr-battery-ram-specs
A couple of specs you’ll never hear mentioned at an Apple keynote or find on the company’s website are an iPhone’s actual battery capacity and the amount of RAM inside each phone. Thankfully, Apple has to file this stuff with government regulatory agencies like China’s Ministry of Industry and Information Technology (TENAA).
The company has done just that for its latest iPhones, and you can find links to the filings at MacRumors. The important thing is that we now know the battery and RAM specifics for the iPhone XS, XS Max, and XR.