The simple answer to why trees in rainy places have big leaves is, "Because they can." There are both positive and negative effects on trees with big leaves, and the environment of a rainy location maximizes the positive reasons to have large leaves, while minimizing the negative aspects that go along with them. Big leaves are well suited to rainy climates, so many trees that grow well in rainy places have them.

The leaves on a tree fill a variety of purposes. They are the primary location for photosynthesis, the process trees use to convert sunlight and carbon dioxide into chemical energy, which they use to grow, and oxygen, which they release back into the air. The leaves also collect rainfall and direct it to the tree's root system. The environment of a rainy location encourages the trees there to grow big leaves.

To grow big leaves, a tree needs plenty of nutrients. Wet soil encourages decomposition of plant and animal matter in the soil, which in turn means that the soil of rainy places often contains a lot of nutrients. This rich soil allows the trees that grow in it to more easily produce big leaves. The soil in a dry location would have slower decomposition, so nutrients may be harder to come by.

Trees also need ample water to grow, and a very rainy location obviously has plenty of that. The big leaves catch the rainfall and funnel it down to the tree's root system, where the tree can absorb the water. Leaves also release water vapor into the air. In dry climates, trees with large leaves would quickly dry out. In rainy climates, trees can support large leaves because they easily replace the water the leaves lose to the air.

Very rainy locations tend to have a lot of cloud cover. The more cloud cover an area has, the less direct sunlight it will get. Trees need sunlight to perform photosynthesis, and having larger leaves in a rainy climate allows trees to absorb more of the limited sunlight. The larger leaves give the trees more surface area to catch the sun's rays. If the trees in a rainy climate had smaller leaves, they would get relatively less sun.

Trees in rainy places have tend to have large leaves, and trees in dry places tend to have smaller leaves, because the size of the leaves need to fit the environment in order for the tree to survive. Trees in rainy places have the water and nutrients needed to support large leaves. The leaves benefit the tree by providing a larger area for photosynthesis to take place. The rainy climate also means that the tree can afford to lose the water vapor that comes from the larger leaves. In other words, trees in rainy places have large leaves because they can.