Q.Why Are My Oranges Smaller This Year?
On an otherwise healthy dwarf orange tree that has produced very large navel oranges in the past, this year has produced a bumper crop of very small fruit ( about the size of little tangerines).
Certified GKH Gardening Expert
You didn't mention whether this tree is containerized and I'm wondering if it needs to be repotted. Other possibilities are that it may be experiencing a zinc deficiency or it may have contracted some kind of fungus or pest. Inspect the leaves and stems carefully for microscopic invaders. You may want to treat the tree with neem oil. You may have a larger problem, but without knowing more details it would be tough to speculate.
It's very possible that you simply need to thin the tree and remove the small fruits to allow for the larger ones to grow.
Here are some articles that may help:
https://www.gardeningknowhow.com/edible/fruits/citrus/fruit-thinning-citrus-rees.htm
https://www.gardeningknowhow.com/edible/fruits/fegen/reasons-for-small-hard-fruit.htm