a small person cannot understand a great person's ambitions
A yojijukugo proverb meaning that a petty person cannot comprehend the grand aspirations of a hero. Often used to express that only a great person can recognize another's greatness.
燕雀鴻鵠 means that a small-minded person cannot understand the ambitions of a great person.
From Chinese classical literature; the phrase contrasts small birds (sparrows and swallows) with a great bird (a phoenix or swan) to illustrate the gap in understanding between the petty and the heroic.