to get a big head 极度自负（成语）

To get too high an opinion of one’s ability, power, or importance; to become arrogant.

例句
After his promotion at work, the young man got a big head and started telling everyone he was much more talented than the boss.

在得到晋职后，这位年轻人变得极度自负，开始逢人就说，他的能力远超过老板。