I think I figured it out. The 'full resolution' option appears if you choose the Web Browser instead of the desktop app to transfer the images. I'm seeing larger file sizes now with this option. I don't know yet if they all came across at the max res available -- of 1300 images, 76 are still at sub-1MB in size. But comparing the previously transferred versions to the new ones, most are ten times larger in file size. So this is a better starting point. I hope that helps someone with the same problem.