too big for one's britches

Too big for one's britches describes someone acting more important or capable than they really are—overconfident, arrogant, or presumptuous beyond their actual status.

The image is of a child who has outgrown their pants, or someone puffed up with unearned self-regard: Ever since his promotion, he's gotten too big for his britches.

Also *too big for one's boots*, especially in British English.