The screenshot is taken at the same resolution as your iMac display and therefore retains the sharpness of HiDPI on Retina displays. If you want your screenshots to be at a non-HiDPI resolution, either scale it to 50% and export through Preview or change your resolution by going to System Preferences > Displays, Option-clicking on "Scaled" checking "Show low resolution modes" and changing to "2560 x 1440 (low resolution)" before you take the screenshots.