The chapter ends on a warning that reads like a comment on its own advice. Easy promises are bound to be little trusted, and much easiness is bound to bring much difficulty; so the sage treats things as difficult and ends with no difficulty.
That appears to contradict everything before it. The chapter has just argued that great things are done while they are small and difficult things while they are easy, and now it says that easiness produces difficulty and that the sage regards matters as hard.
The two are compatible and the resolution is in who is doing the estimating. The earlier advice concerns when to act, and says the moment is early. The closing warning concerns how to assess, and says the estimate should be pessimistic. A person who acts at the right moment because he has taken the matter seriously is doing both. A person who defers because the matter looks easy is doing neither, and will meet it later at full size.
The clause about promises makes the same point in a social register. An undertaking given lightly is discounted by whoever receives it, which is a claim about how commitments are read rather than about whether they are kept.
Chapter sixty-four continues the argument in the next breath and supplies the images: the tree from a shoot, the terrace from a basket of earth, the journey beginning underfoot. The two chapters are close enough in subject that they read as a continuous treatment of beginnings split by a chapter division, which is one more instance of the divisions being an editorial layer.