That depends on how the used recovery program worked: instead of relying on the file system entries, it could have searched the entire memory card for image data and converted that to files. In that case, it's entirely possible that it "recovered" the thumbnails embedded in your real image files only.
If you haven't changed the actual content of your card with any tool yet, i'd recommend using a different tool, unfortunately i don't have personal experience with recent tools to recomend one. Undeleting files on FAT filesystems used to be a fairly trivial procedure, as nothing is actually removed, just marked as "deleted".