D,
Very interesting question (we've wondered it too over the years), but I'm with Roy that for now it would be difficult to study this.
Tank studies have shown that food and temp have very strong influences on ultimate size, and these variables are different for most octos in the home, as well as in the wild. Genes are also important, but would need to be studied in a very controlled generational study. Otherwise it would be very hard to control for multiple paternity in (many?/most?) cephs.
To look for tank effect on size, you'd have to compare your own octo's growth with its siblings in the wild- at its home site, somehow controlling for food intake and temp, and ideally having multiple people doing this. Any difference may be tank-related (e.g. changes in stress and activity), but that would only hold for those individuals. Other ceph keepers may have bigger tanks (more exercise) or poor conditions (higher stress levels). So the ultimate impact on animal health is hard to know, but an interesting question nonetheless!