too big for one's boots

too big for one's britches also too big for one's boots (conceited, self-important; thinks one is more important and powerful than he / she really is) — зазнаться, заважничать; задаваться; воображать; строить из себя; раздуться от собственной важности; (высоко) нос задирать

Example 1: Ever since his promotion, he's become too big for his boots.

Example 2: There's no talking to Jill anymore — she's just too big for her boots.

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This metaphoric idiom alludes to becoming so "swollen" with conceit that one's pants or boots no longer fit. (Answers.com)

see also
[full of oneself]
[put on airs]
[put on the dog]
[high horse]
[airs and graces]
[понты]