There is existence prior to heaven and earth being born.

Quiet and still, independent and unchanging, rotating without end, it is the mother of the world.

I don’t know its name, name it as Dao, and remotely name it as Big.

-> The nature of the Dao.

Big and therefore everlasting, everlasting and therefore far-reaching, far-reaching and therefore returning (to its source).

-> Real biggest does not only pertain to size and power, but also of its everlastingness, far-reachingness, and its at one with the source.  Look at great teachings of all great founders of major religion, founders are gone but their teachings are still presence thousands years after around the globe.  Big, therefore, is everlasting, far-reaching, and exist independently as the source.

Therefore Dao is Big, Heaven is Big, Earth is Big, and King (Man) is also Big.

-> What in the world is Big then?  Dao is, heaven is (relatively big and far-reaching), earth is (relatively big and everlasting), and extraordinary man is (those realized the Dao).

These are the four Big in this universe, and King (Man) is one of them.

Man follows the Earth, Earth follows the Heaven, Heaven follows the Dao, and Dao follows its own.

-> Ordinary man follows the rhythm of the earth (the planet and its laws), earth follows the rhythm of heaven (the solar system and its laws) , heaven follows the law of the Dao.  What then, does the Dao follows?  It is what it is, beyond the notion of something following others (which implies definitions and separation).  And therefore is far-reaching, everlasting, and independent beyond all.

* * *

Chapter 25 Discussion:

What is Big?  When you ask ordinary people, they might say, bus is big, money is big, corporation is big, city is big, or country is big.  When you ask what else is even bigger? They might say, earth is big, sun is big, or universe is big.  So does ancient Chinese people, they thought heaven and earth are big.  What is the Biggest of all? Laozi said, Dao is.  Why? Because it exists prior to even heaven and earth, which is far-reaching, everlasting, and at one with one’s source.  How come Man is described as one of the big fours?  Because we have the potential to realized the Dao.  Such is one of the blessings of being human.

Bilingual text of Chapter 25: https://www.lisiming.net/philosophy/chinese-philosophy/daoist/daoist-philosophy/dao-de-jing-core-33-chapters/ddj11-3/