Maybe this is happening to your tree, maybe you see your neighbor’s tree succumbing to this problem.
What are you seeing?
You are noticing that the leaves on one side of your tree are smaller than on the other side, and it is puzzling you. You are tempted to call a tree specialist, but, before you do that, do this one thing for me, it could save you a bunch of money.
I would like you to dig around the base of the trunk starting on the side where the leaves are small; my guess is you are going to find a root about an inch and a half to two inches in diameter pressing up against the trunk and girdling that side of the tree. This is your problem and cutting it out is the solution. Next I suggest you water your tree once every three days for a while just to help it out. This root has begun to crush the cambium tubes that carry water up the tree, and it is going to take some time to repair them. Unfortunately I can’t guarantee that your tree will recover, I don’t know to what extent the tubes were crushed; but doing nothing would have insured the tree’s demise.
It seems that this occurs when a tree is about 12 to 16 inches in diameter; and it would be a good idea to dig around the base of these trees from time to time and eliminate future problems.