Why are only the thumbnails recovered?

Every once in a while when doing the digital image recovery, typically off a memory card, the small image previews (called thumbnails) are recovered OK, while high-resolution images are damaged.
There is a phenomenon called “file fragmentation”. The fragmentation is said to occur if the file is placed on the disk in multiple non-contiguous fragments.  The graphics showing the fragmentation can be found at the Photo Recovery Limitations page of the Digital Photo Recovery Guide.