If you have a specific size chosen in responsive design mode, that size will be scaled by the current browser zoom (but the numbers won't update). I'm not sure if this is intentional? It's especially confusing after bug 844604 though, where you might have zoomed the page out to restore a 1:1 pixel ratio and then all your sizes in responsive design mode are larger than they appear/are presented at, causing CSS media queries to fail. So for example, if you have a size of 640x480 in responsive design more and your desktop has a 150% scale, then you zoom out back to 100%, you end up with a size of 960x720 - but the UI still says '640x480'. I think it would be nice/expected for the resolution to be maintained when changing the zoom, but for the border to shrink.
Just to pre-empt any over-zealous duping, this is not bug 758011 (which would also be nice :))
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