Most important part of the question is what software are you using to view the images. How do you know that the images aren't the same quality? I don't know about Macs, but on a PC all you need to do is right click on it, choose properties and you'll see the image size of the file.
Copying a file should not change the quality of the file itself. It sounds more like your image-editing software isn't showing you the full resolution of the image. Or, it could be an import setting, but not likely. If all you're doing is ctrl + c and then ctrl + v to your computer (to copy and paste), then there cannot be a difference between what is on the memory card and what's on your computer.
With this in mind, I would venture to guess that the software you're using to view the image on your computer is showing you a low-resolution JPEG preview. Lightroom, for example, does this. It's much faster to show thumbnails of the images instead of previewing full-resolution images. You can often change the quality settings for each thumbnail preview from low, med to high at the expense of taking up more time and disk space.
I would encourage you to not use the supplied Apple software and to just use Adobe Lightroom which is the industry standard for image editing, sorting, and cataloging digital images.