The screengrab taken on the “normal” unscaled display has the correct pixel dimensions along with a DPI (Dots per inch) of 72, which has been the standard DPI on Macs for a very long time. The screengrab taken on the Retina display, however, is much larger, with a proportionately higher DPI. This makes sense, due to the way OS X handles scaling on the Retina display.
So, even though the image appears “normal” size on a Retina screen, it’s actually being rendered at the higher DPI / larger size and scaled down. When the OS X screencapture utility is used to take a screen grab, it captures the image at the higher DPI and resolution. And when that image is uploaded to be viewed on the web, it is way too big.
Seems the solution is http://www.retinacapture.com/