The complete sentence is: Confucius climbed the East Mountain but was small, and climbed Mount Tai but was smaller than the world. So this sentence should be said by Mencius, and he is talking about Confucius.

It means that Confucius boarded the Dongshan Mountain in the state of Lu, and the whole state of Lu had a panoramic view; Confucius climbed Mount Tai, and the world was unobstructed. On the surface, it refers to the height of Mount Tai, but in fact it refers to people's horizons. Perspectives should constantly seek breakthroughs and surpass themselves, and look at the ever-changing disputes in the world with a detached attitude.