Do trees planted on dwarfing rootstocks produce small apples?

Without question, a dwarfing rootstock can reduce vegetative growth, resulting in a tree with a smaller trunk, shorter height, narrower canopy, and lower root volume. These rootstocks, however, do not reduce size of leaves, flowers, or fruit. Nearly all NC-140 apple rootstock trials have evaluated rootstock effects on fruit size, and, in some cases, rootstock did in fact affect size. In no case was the relationship between the degree of dwarfing and fruit size such that the more dwarfing the rootstock, the smaller the fruit. In fact, many of the dwarfing rootstocks, for example M.9, actually result in the largest fruit size.

Answer provided by Wes Autio, University of Massachusetts, Amherst.