Display settings are not all the same on iOS devices, it depends of the size of the screen.
Default display setting on iPhones is Medium.
Default setting for iPad Pro (12.9) is custom resolution.
Default setting for the rest of the iPads is auto-fit low.
The reason why the default setting for iPhones is Medium is because the rendered desktop items are very small (start menu, files, start menu button, …) and it is difficult to use in case of auto-fit on iPhone.
The decision was made to make the default viewing size set to Medium for the smaller screen devices. This can be changed within the settings of the Workspace App for iOS.