better to be the leader of a small group than a subordinate in a large organization; better to be a big fish in a small pond
A yojijukugo proverb. Literally 'rooster's beak, cow's rump'; the rooster's beak is small but leads, while the cow's rump is large but follows. Used to encourage taking a leadership role even in a modest setting.
Rather than being a rank-and-file employee at a big company, it's better to become the president of a small company. That's exactly 'better to be the head of a rooster than the tail of a cow.'
From the Chinese historical text 'Records of the Grand Historian' (Shiji). The phrase contrasts the small but leading beak of a rooster with the large but following rump of a cow.