IDIOMS

too big for one's britches

(idiomatic) Disturbingly confident, unacceptably cocky.

Example: 1989, Edwin M. Reingold, "Hard Times for Teflon Tom," Time, 22 May:
  A diffident, dedicated man, Bradley seemed the personification of rectitude. He never got too big for his britches.
Used other than as an idiom: Too large to fit into one's pants.