Leaves are the plant's "solar panels". Down in the lower canopy the light levels are lower. In this low light environment a plant produces larger leaves to capture more of the limited light intensity.
Also, as a tree begins to grow it produces growth that is known as "juvenile". As the tree gets older and larger it reaches a stage where "mature" growth is produced. This is why a young pecan tree does not bear pecans and an older one does. Leaves on juvenile growth are often larger than leaves on mature growth. Sometimes on a fairly young tree this lower juvenile area produces larger leaves than the mature leaves higher in the canopy.
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