Half the fruit have plum curculio damage ie the typical crescent shaped scar on the skin. Female adult curculios are responsible for these scars. They cut into the fruit just under the skin and insert an egg into each slit. The egg hatches into a larva that eats its way through the tasty plum flesh that was supposed to be feeding me. This larval feeding can cause the fruit to drop prematurely. On my tree, I'm seeing a lot of crescent shaped scarred fruit that seem to be growing with no intention of dropping. These could be the ones that grew so well they crush the larvae in them. Curculio-killing plums!
The plums are delicious and I don't mind sharing some with the curculios. But I'm still going to destroy all fallen fruit so the larvae can't finish their life cycle and infest next year's fruit or something.