Leaves have a large surface area to maximize the sunlight they receive for photosynthesis. Generally, leaves that are in the lower canopy have larger surface areas than those in direct sunlight because they have fewer opportunities to capture sunlight.
However, large leaves increase heat absorption and water loss, which can be quite damaging to plants. In deserts and other areas where water conservation is more important than sunlight acquisition, leaves with smaller surface areas are common. These plants often have needle-shaped leaves with thick cuticles to prevent water loss. Plants perform a balancing act to keep the surface area of the leaves large enough to get adequate sunlight without gaining too much heat or losing excessive water.