Dear Kathy,

Thank you for contacting Extension with your question.

A probable cause of the small leaf size may be moisture - too little or too much. Moisture stress can interfere with nutrient uptake, which may or may not cause abnormal leaf development. 
Other factors can also cause unusually small leaves.  It could be generally attributed to a combination of factors including drought, pollution, de-icing salts, sidewalk/driveway/road construction, excessive fertilizer or pesticide use, root damage, etc.
Continue to monitor the health of your tree.  Look for signs of borers, damage or discoloration of the bark, or dieback on portions of the tree.  Please contact us with any further concerns.

Best regards,
Cindy G.