bah! If you read the lurker's guide a lot of people have plenty of success with O. S. in something small.
Even more people have them die after a few months! The issue isn't so much whether you can keep a large O.s in a 5 or 10 gal system - you can. The real issues are whether you can maintain the health of the animal and/or prevent a sudden collapse of the system resulting in the death of the animal. It is nice to see the animals roaming far and wide around a 300 gal aquarium. In nature large males often travel tens of meters away from their burrows searching for mates and food, but such movement isn't really necessary for the health of the animal. They can get plenty of exercise rearranging the substrate and bashing on shells and tank dividers. (I would note that an O.s that digs all of the time is not particularly satisfied with its surrounding. However, just because you engage in excessive home improvement projects does not mean that you are psychotic - perhaps just a bit neurotic!)
The real issues with a very small system center on water parameters and the ability of the aquarium to resist perturbations. O.s. living in these systems are more prone to shell disease, molt failure and raptorial appendage loss, and death do to spikes in ammonia and nitrite. That is not to say that these aquaria cannot be maintained with good quality water, but they require more attention and when things go wrong they can degrade very quickly. A buried molt skin or chunk of uneaten food can kill a large O.s in hours. Since most small systems do not have uv sterilization, pathogens can build up and produce really nasty cases of shell disease. Small tanks are more prone to temperature and salinity fluctuations. The list goes on.
Also, unfortunately many of the people setting up small systems are relatively inexperienced keeping aquaria and are poorly equipped to manage these more unstable aquaria. Sure there are enthusiastic first timers who lay down a few thousand for a state-of-the art 300 gal tank and don't know that you have to add salts, but many O.s. are going home from the shop to a tank that hasn't even cycled
Anyway, these are some of the reasons I don't recommend a small aquarium for a large O.s.
Roy