I'm working on my external (non-Retina) monitor, with my Retina MacBook as a secondary screen next to it. Not uncommon, I believe.
When I open a document Photoshop displays it at Retina-size (i.e 50%) for a split-second before zooming in to 100%. It keeps the window size at 50% though. Since this can get a little annoying Photoshop should adjust the window size to show the full document instead.
(This happens with "Zoom Resizes Window" turned On and Off, as far as I can tell.)
OK. It sounds like we're talking about two different things. This issue reported by the original poster where the window is smaller and has scroll bars when it should be expanded and show the entire image has been fixed. The behavior described in later comments is covered by this topic: http://feedback.photoshop.com/photosh...