The jpg files will open in whatever app you specify is the default. In the Finder window, right-click on a file (if you have a 2-button mouse), or just control-click the one button till a contextual menu comes up. Choose "open with" and then the appropriate application. You can say to choose Preview for jpgs as that is an easy way to view them without going to the time and RAM commitment of opening Photoshop.
As to file size, 60-70 MB is not bad. My multi-layer LOs wind up between 90-110 MB as psd files. So, you've done nothing wrong. That is just the reality of it, and why we generally need EHDs to handle the files. Once you've made sure the LO is what you want to keep for printing, and won't need to do any more alterations, you can save a final flattened jpg at high resolution for your archive, and get rid of the psd file. Or not... depending on your level of paranoia about crashes, file losses, etc.