This happens because the screen output image on Macs with Retina displays have double the number of pixels, to take advantage of the system’s greater pixel density for a crisper image. Therefore, when you take a screenshot, you get an image containing double the number of pixels than the same image from a non-retina Mac.

Most programs used to open this image should account for the image size using its embedded PPI settings to properly display it; however, sometimes this may not be properly accounted for. For example, if you take a screenshot to the clipboard and then make a new image from the clipboard in Preview, each pixel in the image is presented by a logical pixel on your Mac. Since there are double the number of pixels in the image, this means that logically the image shows up at double the size along each axis, or four-times larger in total.

This error is actually a problem with Preview’s handling of the image, which should have an embedded DPI setting of 144 (double the classic 72 DPI). If Preview were to manage this correctly, then the image would be represented in a proper size in inches, regardless of the number of pixels along either dimension. However, this is often not the case, either in Preview or in some other programs, including some online sharing services.