Great question! Before I get into it, here's a really quick guide on exporting:
To answer your question, however: iOS has a concept of a virtual point. I found this great explanation of the difference between pixels and points here: http://blog.fluidui.com/designing-for-mobile-101-pixels-points-and-resolutions/ It's a great read, and I think really valuable for understanding what I'm going to say next.
When XD outputs for iOS, it's going to output at 1x, 2x, and 3x resolutions. The 1x will be for your older screen sizes, 2x will be for up to iPhone 5s, and 3x will be for your newer, higher resolution screens. The actual size it outputs, though, is dependent on how you designed. If you design with the default artboards we have (which are, if you notice, half the size of what you're used to), you can leave your export settings to "designed at 1x," and it will export at 1 virtual point = 1px (aka 1x), 1 virtual point = 2px (aka 2x), and 1 virtual point = 3px (aka 3x).
However, if you duplicate the artboard size you're used to in Photoshop, you should select "designed at 2x" in the export panel in XD. This will output 1 virtual point = 1/2 px (aka 1x), 1 virtual point = 1px (aka 2x), and 1 virtual point = 1.5x (aka 3x). You'll get the same exact pixel size in your output in both cases, but in the latter, your artboards in XD are much larger.
Hopefully it all makes more sense to you now! Let me know if you have additional questions and I can try to answer.
Hi, i still don't get it. Currently I'm using the default artboard iphone5SE not because I'm using it for iphone5SE but because I'm building mobile site which is start at 320 width. I tried all the export options, whatever I picked (IOS, Web, Android) still the biggest one is 320 width. I don't understand the logic for example if I choose web, why when it's @2x (the highest) then 320 consider as the highest? This is not the logic like in Illustrator. In illustrator, it's clear when you scale up @2x then it will multiple 2 from the original artboard. So why should I do now? Create a new one with the high resolution that I need? Or is there any other option for it?
I think what you want to do is set to "Designed at 1x." It should output what you're looking for at that point.