In fact, 5 gallon batches are possible with the 20 gallon mash tun, however, expect to loose a bit more heat as a result of the larger head space. The most important thing to compensate for the large amount of head space is to make sure that the mash tun is thoroughly pre-heated, that way the vessel doesn't inadvertently draw heat away from the mash. Furthermore, try to avoid removing the lid during the duration of the mash rest, since all of the heat built up in the head space will be released.
Users should expect a degree or two more in heat loss when compared to mashing in a 10 gallon tun as a result of the aforementioned factors. Head space, and thermal mass are the two largest considerations when working with insulated vessels. The larger the thermal mass, the smaller the head space, so an insulated vessel will be exponentially more effective if used with larger volumes.
Lastly, an old trick that brewers use is to lay a few sheets of tin foil lightly over the top of the grain bed to help condensate reenter the main mash volume. It takes two seconds to do and adds another layer of insulation to the top of the mash.