How can two trees planted
at the same time be different sizes?

There are several possible reasons why these two trees have grown to different heights. First, each tree may have inherited different characteristics from the parent trees, causing one to grow faster than the other. (In many kinds of living things, including humans, sisters and brothers can be very different from one another, even though they have the same parents.) Second, one tree may be in a spot where it gets more sunlight, water, or nutrients than the other. And third, the shorter tree may have insects, fungi, or other pests living off it, which would make it sick and slow to grow.