The small amount of sulfite is that which is only a little more than is sufficient to form a Metol sulfonate from the oxidation product of Metol, according to data from the Theory of the Photographic Process. That same data showed that ascorbate, at pH below the point where ascorbate becomes an active developing agent, is equivalent to sulfite, mole for mole, for the purpose of counteracting the retardation by the oxidation products of Metol, although they work by different mechanisms. I wasn't expecting either it or the sulfite to be a preservative or a grain reducer. I think it's better to think of ways to avoid increasing grain, since the image is made of grains in any caase.