Rosalind definitely overshadows Celia throughout the play. Rosalind is a major character, while Celia is only a minor character who exists partly to provide opportunities for dialogue which informs the audience of important plot developments, such as the fact that Rosalind is in love with Orlando and that Rosalind intends to join her banished father in the Forest of Arden. Celia's father recognizes that Rosalind overshadows his daughter. He tells her: "She is too subtle for thee; and her smoothness, / Her very silence and her patience, / Speak to the people, and they pity her. / Thou art a fool. She robs thee of thy name, / And thou wilt show more bright and seem more virtuous / When she is gone." It is because Rosalind is the stronger character that she chooses to disguise herself as a man when the two young women travel together. Rosalind makes all the decisions and acts as protector of Celia. Both characters, of course, were played by young males. No doubt the one who played Rosalind/Ganymede was older and taller than the boy who played Celia.
well yeah i guess because oliver falls in love with rosalind instead of celia