How could Sha'ul's garments fit David? Everyone else's head reached no higher than Sha'ul's shoulder (9:2)!
Rashi (from Vayikra Rabah 26:9): One who is made king, becomes tall. From when David was anointed, his body improved. Sha'ul noticed [that his clothes fit David, and sensed that David will become king], and was envious; David noticed this.
QUESTIONS ON RASHI
Rashi writes that Sha'ul became envious when he saw that his clothes fit David. He should have noticed beforehand that David was as tall as him!
Yalkut Shimoni (Shmuel 125): Once he wore the garments, and he was proper for kingship [also regarding his clothes, he grew], and the clothes fit him.
Etz Yosef (on Vayikra Rabah 26:9): Sha'ul did not notice until David wore them. He expected that they will be too big, and David will take them off; he offered merely to be gracious. Or, he thought that David will raise them [from dragging on the floor] via a belt.
