Is the question about beer or wine (or another ferment)?
It is perfectly possible to brew 1 gallon of liquor in a 5 gallon container. The main concern is the air over the brew which, some might say, causes "oxidation" and thus produce off flavours in the brew. If the fermentation was started vigorously (active yeast made into a starter solution) then the CO2 being produced should quickly blanket the surface of the fermenting liquid and that would act as an reasonably effective oxygen barrier. I personally ferment beer in a 50lt pastic tub with the lid clipped on but just lifted in one place to allow pressure to escape. There must be 8-10 litres of air over the brew (and no airlock) but I have experienced no bad effects so far.
As far as I know fermentation does not need any "back pressure" - most fermentation is carried out at or just above atmospheric pressure (via a bubble trap). Allowing pressure to build up during primary fermentation is usually regarded as a bad or even dangerous thing.
Racking wine or beer off the trub/yeast/detritus is a reasonable thing to do and is said to improve the flavour (less yeasty taste, they say) and it may also help the brew clear down ("drop bright") but is done at the brewers discretion. There is no absolute need to rack the brew into a second container, it is done only to improve clarity and taste. If your brew is clear and tastes good in the primary vessel then there's no need to do it. If one decides to rack off the brew then a container than can be mostly filled may be a better option to one that can be partly filled or one that leaves a large air volume above the brew. IMHO as both beer and wine can be brewed "in air" the presence of air in itself need not be overly worrying in a closed container.
All of the above is, of course, opinion. The best thing to do is try a brew and see. Start with what you have and if that works - success \o/. If not then try the smaller jugs or scale up the production process.