The word 长大 is formed by combining two characters that depict a direct, cause-and-effect sequence. The first character, 长 (zhǎng), means "to grow" or "to increase in age/size", representing the ongoing process. The second, 大 (dà), means "big" or "large", representing the resultant state. Together, they create a natural conceptual flow from the action of growing to the achieved condition of being big, perfectly encapsulating the idea of "to grow up".