Do dwarf trees have smaller size fruit? I have dwarf plum trees. The fruit is sweet and tasty but small.
Did you thin the fruit? Dwarf fruit trees refer to the size of the tree not the size of the fruit.
Do thin to 6 inches. Doesn't seem to matter as fruits 6 inches apart no bigger than those that I didn't get to thin, which are in a cluster of 3 or 4.
Generally the rootstock does not greatly affect the fruit size. It can make a small difference. Some growers don't like using OhxF333 pear rootstock for some pear varieties due to smaller fruit. It was never a big enough difference for me to notice. Most commercial apple orchards planted today are on dwarf stock; given the importance of large fruit in the market I'm sure those stocks don't make the fruit any smaller, otherwise they would not be used.
Small fruit can be due to many reasons. Lack of water in spring, or the season before, too much water in the spring, incomplete pollination (iir this i the most common) due to lack of good pollinators.......
Do you know what kind of plum they are? It could just be as simple as needing a pollinator and some light thinning of the fruits
The variety could be the key. Some are smaller than others.