HELP! Photo resolution sucks after unarchiving zipped back-up to JPG

I unarchived all my photos from my external hard drive with no problem. As I was clicking on my unarchived photos which have been converted to JPG they are all coming up as a preview only; just super small and low resolution. I've tried compressing them again, opening them with photoshop and iphoto, but they still are the same size as the preview. ALL of my photos appear to have been shrunk permanently. Should I have unarchived the pics differently? I just clicked 'open' and my mac automatically unarchived them beautifully with archive utility. I'm not sure if I am just unfamiliar with my mac and am doing something wrong or if the pics are ruined from compressing them. Help if you can!