A difference between iPhone XS and iPhone XS Max is Display Zoom, an accessibility feature that makes content on the screen appear larger than the standard resolution. This was introduced on iPhone 6 and iPhone 6 Plus, but was mainly absent on iPhone X. The display function is still absent on iPhone XS, but the larger iPhone XS Max version restores it.
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Zoom view can be enabled in the Settings app under Brightness and display of the screen. By default, more content appears on the screen and zoomed shows larger controls for content. Show zoom makes elements on the screen more readable with larger tap targets. All modern iPhones support the adjustment of the text size where dynamic text is supported.
Dynamic Zoom showed a dramatic difference on iPhone 6 Plus and other 5.5-inch phones, but was less subtle on iPhone 6 and other 4.7-inch phones. That's because Display Zoom presented a version of the operating system, designed for much smaller screens, but scaled up. The effect was the same as an app designed for a 4-inch phone like the iPhone 5 on a larger screen without support for full resolution.
iPhone X and iPhone XS have a larger 5.8-inch display, but a different aspect ratio than iPhone 5 and iPhone 6-style phones. For this reason, there are no smaller versions of apps that can scale up easily for Monitor zoom.
iPhone XS Max iPhone XR
iPhone XS Max has a larger 6.5-inch screen and the same aspect ratio of iPhone X, so apps designed for iPhone X and iPhone XS can be scaled up in Display zoom on iPhone XS Max.
This probably means that Display Zoom is also displayed on iPhone XR. It features a larger 6.1-inch screen with the same aspect ratio as the iPhone X (and the same practical resolution as the iPhone XS Max, only 2x instead of 3x).
The Display Zoom effect is visually less dramatic on iPhone XS Max (and probably iPhone XR), but the jump from 4-inch screens to 4.7-inch and 5.5-inch screens is more dramatic than 5.8-inch screens 6.1-inch screens and 6.5-inch.