It can be interesting, to get started, to make mead in quantities smaller than the one used in the recipe (what do I do with these 20 liters of vinegar or of bacteria culture?)
Small amounts can be used for experiments: instead of making 5 gallons of 1 mead, it can be better to make 5 meads of 1 gallon each. This allows to try new yeasts and honeys. It is not required to make 20 liters of mead with some exotic honey whose taste and cost are unusual.
This also allows to get used to fermentation techniques without logistics troubles like handling more than 10 liters of boiling water or a carboy weighing more than 20 kg (40 lbs).
Small amounts also have drawbacks. The surface area/volume ratio increase and so does the risk of oxidation. Some of the losses are independent of the volume (sampling for tasting or chemical analysis as well as losses due to rackings are proportionally higher with low volumes). Cost and labor will also be greater: 1 dose of yeast, 1 stopper, 1 airlock are necessary for 1 gallon as well as for 5 gallons. Measurements also become a problem: the smaller the amount the larger the relative mistake. Being off by 1 g out of 4 g does not have the same consequences as being off by 1 g out of 20 g.