When taking a screenshot and pasting it into XD the pasted image is 2X larger in dimensions

XD imports all files at 1x, except for JPEGs which it imports at 2x. Your 1500x1000px screenshot is actually 2x which means the file is 3000x2000. XD is treating it as 1x, so you'd have to scale it down to make it 2x. There's nothing in pixel-based files to tell XD whether they are 1x or 2x, so XD has to make some assumption as to how to import it. I do wish though that it assumed all files were 2x (not just JPEGs).