A tree grows by branches splitting into smaller branches, and the combined thickness of the new branches equals the thickness of the branch they came from. Because of this conservation, every branch is thinner than the one it divides out of, so the trunk is thickest and the twigs thinnest.
Rather than copying a tree's outline, you reconstruct it by applying its generative rule: start from the trunk and repeatedly split each branch into thinner ones at consistent angles until you reach the twigs. The specific angle, curvature, and rate of thinning you choose produce the different species and shapes.
A tree's final shape is not arbitrary but the visible trace of how it responded to its conditions: wind, sunlight direction, soil, and obstacles bend and redirect its growth. Reading a tree's form therefore tells you about the forces that acted on it as it grew.