Thanks for your reply. I just notice that the problem is not recurrent. Yesterday, some of the pictures were no bigger than a postal stamp on the Retina display. Today they seem normal although I notice that they appear relatively smaller (like everything else) on the Retina display than they appear on my iMac. That being said, I compared the resolution of a bunch of pictures on my Powerbook Retina with the very same pictures on my iMac and some of them have a different resolution for exactly the same weight in megs!
As for your other question about how I transferred my Library from my old MacBook to the new Retina one, I just used the Mac program for such transfer using an ethernet cable.
If the pixel dimensions of the photos have remained the same then the diffrerences in appearance on your Retina display is due to the many more pixels in that display. Have you applied all of the updaters for the MBP Retina display?
What do you mean by resolution? DPI? DPI does not come into the equation until you go to print a photo. This webpage explains DPI: The Myth of DPI
Thanks again for your help. To answer your question, yes I have the latest update on my MBP Retina. And by resolution I do mean the number of pixels and not the dots per inches for printing purposes. The problem has not reoccured since my last message, although the pixel count does vary from one computer to the other, which may have happened when I copied the pictures. I also understand that sizes normally appear smaller on a Retina display. Thanks again.