I had the same issue and found out what it was, though there is not a perfect solution for me yet. Please correct me if I am wrong.
The Problem occurs when you have a scrolling Artboard. You have to specify a "Viewport Height", which is (when you first activate Scrolling) usually set at 1080 for a regular 1920x1080 Artboard. When you view this Artboard in the web, Adobe XD sees, that your Artboard does not fit on screen because it is heigher than your browser window. This is true because you have to account for your OSes Taskbar and the browser top (Address Bar, Bookmarks, etc.). Thats why Adobe scales the preview down, thus giving you white bars on the left and right.
A solution to this is to query your Browsers actual viewport height. You could do this via the JavaScript console by typing
"window.innerHeight". Assuming you have Chrome with a Bookmark bar, this will result in a value of "937px". If you set the "Viewport Height" to exactly this value, it will display without bars and will fit the browser viewport perfectly.
Of course display varies for people that do not have an active Bookmark bar or different Browsers, so here is a Feature Request for you Adobe. Let us simply input "0" for Viewport Height, and ignore scaling according to height, if this is set.
Hope this does help someone, to at least preview their prototype fullscreen.