https://www.techradar.com/news/why-you-may-need-a-new-iphone-xs-case-after-all

The just-revealed iPhone XS might look just like last year’s iPhone X, but it turns out they’re not exactly the same. The new smartphone’s camera housing is ever-so-slightly bigger, which means it might not fit if placed in a case made snugly for the iPhone X dimensions.

The longer side of the camera housing on the iPhone X is 24.13mm, while the new iPhone XS was enlarged to 25.50mm (the shorter “width” was only enlarged by .01mm), Macotakara found. This is supposedly because Apple slightly enlarged the wide-angle camera sensor in the dual-camera suite.