too big for one's britches

Example: “Since winning one match, the rookie has gotten too big for his britches.”

Origin: First documented in print in 1835, in Davy Crockett's own written account of his tour through the northern United States, originally as "too big for one's breeches" before the more casually spelled American "britches" took over as the standard form. The image is a simple, physical one: a person who has grown — whether literally in size or figuratively in self-regard — to the point of bursting out of clothing that used to fit comfortably. A closely related later variant, "too big for one's boots," appears independently in both American and British usage from the 1860s onward, applying the identical logic of outgrown clothing to a different article entirely, but making exactly the same point about someone whose sense of their own importance has swollen well past what actually fits.