Hi Matt,
Things that can effect fruit size are: the type of cherry tree it is, as in varietie or unique genetics: the growing conditions, like heat, nutrition and available
water; and how long the fruit has been ripe on the tree under those conditions. If the fruit is a small variety, that grows under poor conditions; then as the fruit ripens, conditions go from bad to worse regarding heat or drought, that can cause ripe fruit to shrivel a bit too.
I hope that helps with the mystery of your cherry tree.