That one actually made relative sense, since it was a subspace compression effect rather than the usual, physically nonsensical explanations for shrinking like molecules getting closer together, or the body losing mass yet somehow retaining enough neural complexity for consciousness. Isaac Asimov explained the flaws with those ideas in his novelization of Fantastic Voyage and his Fantasy and Science Fiction magazine essay about same, and concluded that the only remotely plausible mechanism for shrinking would be a dimensional phenomenon not unlike the explanation given in "One Little Ship." And that episode, like Asimov's novel, also recognized that shrunken people wouldn't be able to breathe normal-sized air molecules and would need miniaturized air as well. So I've always liked the episode for getting it right, or at least as close to right as such a fanciful concept could be gotten.
"Terratin," by contrast, was just ridiculous. It claimed that people were shrinking because of the epsilon rays tightening their DNA helixes, but DNA is only found in the chromosomes in cell nuclei; it's not the sole constituent of the entire body. Changing the shape of a person's DNA would only make it impossible for enzymes to interact with it and would cause cellular processes to stop working. The result would be death, not shrinking.
