Malcolm Owen, Apple Insider:

The iPhone XS is similar in terms of size to the iPhone X, sharing the same length, width, and thickness with last year’s model. While this may mean that most cases made for the iPhone X will fit the iPhone XS, a report from Macotakara notes the slightly larger camera section on the back may be too big for cases where the camera cutout is made with the iPhone X’s dimensions in mind.

It is likely the camera bump has been expanded to accommodate a new larger sensor for the wide-angle camera in the iPhone XS, which features an increased pixel pitch.

If you already have an iPhone X and a case, good to know, but not much you can do about this. But if you are buying a case for the iPhone XS, keep this in mind. Make sure the case is specifically built for the iPhone XS, and not sold as a one size fits all.