Hi Nicole,
I am pretty sure this is a problem we are tracking; We have not addressed it yet, but it is on our list of things to solve.
Artboard documents have an auto-expanding canvas; this is part of the user experience that gives an infinite feel to the document; you can place things anywhere in the view and they should remain visible no matter where you drag them. To help maintain that illusion, we paint the entire matte area with the same color outside the artboards.
We hide this canvas from you, although it is still there. You can see how big it is at any given moment by choosing "Image>Canvas Size...". That size will change as things get moved around, getting bigger and smaller as needed. The hidden canvas size will snap to the next larger or smaller "tile boundary", which is nominally in 1024 pixel increments. This is done for performance.
The problem is that the export code is really unaware of the fact that artboards can grow and shrink the canvas. So when you export an artboard document, it looks at the current canvas size and just uses that. That's why you get that extra white space. This extra white space problem also happens for other things, like document thumbnails. We agree this is pretty annoying, and as I said, it's on our list to fix.
Try this workaround:
1. Get your document all ready to export.
2. Under the Artboard Tool, choose the gear icon (top tool bar, far right) and uncheck auto-size canvas
3. Choose the rectangular marquee tool and cover the Artboard from edge to edge. (Snap helps here)
4. Choose Crop. This will shrink the canvas to just the artboard bounds.
5. Export As..., pick your format, e.g. PNG. and export.
6. Verify no extra white space.
Note that "auto-size canvas" is a document-sticky property and is saved with the document.
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