I've just spent a very frustrating couple of hours trying to get to the bottom of a problem. I've managed to sort it in an around about way, but I want to know a) what the cause is and b) how to stop it. I had to put together a couple of animated GIF files. Using Photoshop 2015.1.2 on a 27" iMac 5K running at 2560x1440 res, I created to file to the exact size required (in this case 978px x 89px). The frames were created in the usual way, saved as an optimised Animated GIF file at the desired resolution. Cmd-i in the finder confirmed that the resolution was correct. Upon forwarding the file to the client (during this episode as tests, I had sent zipped, unzipped, direct emails and file transfers), I was told that the file was coming up small (640px x 59px). That's approx 65% size in pro. Also on my desk I have a 24" imac, running Photoshop CS5 at 1920x1200 res. I copied the GIF file across to that machine, re-saved it and then sent that to the client. It was received at the other end at the correct size. When I'm using Photoshop on the Retina screen, images show at far smaller than they do on my other Mac, but then I'd expect that due to the nature of the screen. However, when comparing the 2 machines as I use them both side by side on a daily basis, I can't really be sure that having the Retina screen is actually positively adding to the productivity process. Nothing appears to be WYSIWYG and if I'm having to ramp down the res to see it, then I can't see the point of having a Retina screen at all. But that's another issue... So - why would one version of Photoshop generate an end result so different from the other?