Asked June 09, 2016, 10:33 PM EDT
Buchanan County Missouri
Sometimes just compacted soil (like those around most homes) or conditions that are stressful can make the tree produce leaves that are small. Also drought (which is a stress). It is difficult to know for sure without seeing it. Call you local Extension agent and see if they can do a site visit.
If you wanted to do something proactively, I might suggest watering with an oscillating sprinkler over the rooting area during times of drought. Water should be applied in the very early morning (4-8am) and you should put down about 1.5 inches each time. Get a butter dish or pie pan to measure.
Also, fertilizer would be something to do proactively as well. Fertilizer should be applied to trees and other woody plants around thanksgiving. Not knowing a soil test, I would apply 10-10-10 at a rate of 20# per 1000 sq ft (2# per 100 sq ft) to the area out from the trunk at least 1.5 times the distance from the trunk to the drip line.