You might also notice that leaves growing closer to the ground, where the forest is shady, tend to be larger and softer. This is because trees are trying to absorb as much light as possible with these shaded leaves. Understory trees such as dogwoods live their lives in low light. They not only have larger leaves to absorb all the light they can get but also grow their branches in flat tiers so their leaves are spread out in single layers to make the most efficient use of the light that filters down to them.
~ Peter Wohlleben
(Forest Walking: Discovering the Trees & Woodlands of North America)