get too big for (one's) britches

To become overconfident in one's importance, skill, or authority; to start speaking or behaving as if one is more important or influential than one actually is. He's gotten too big for his britches ever since he got that promotion. Honestly, John got too big for his britches as soon as pro teams started scouting him. I know you won the election, but don't get too big for your britches—you're only a student council rep!