iPhoto is pointing to thumbnails, not full pix

I had to rebuild a library a year or so ago, with the assistance of Apple help, which is where I suspect this problem started ... in any case:

I just realized that a minority of the 6,000-plus pictures in iPhoto were fuzzy and had strangely low file sizes: 19K, for example. (Many others are crisp and in the multiple-MB size.) I looked into the Photos file and it appears that, at least in the cases I checked (hopefully all!), there is a full-sized photo and also a thumbnail. For some reason iPhoto is pointing to the thumbnail, which I assume means that is the version that would be printed, emailed, and so on.

How can I "point" iPhoto to the full-sized photo and not the thumbnail?