The king of Brobdingnag is a moral giant relative to Gulliver, certainly, and to the Europeans Gulliver describes to him. The king asks for a full account of Gulliver's home -- its government, its history, its way of life -- and he is "perfectly astonished" by what he hears. He concludes that the majority of Gulliver's countrymen are the "most pernicious Race of little odious Vermin" that ever lived due to their violence, cruelty, hatred, and ambition. Gulliver describes the king as wanting knowledge and experience due to his isolation from the rest of the world, and he believes that the king is narrow-minded, when, in reality, the king is precisely right: Gulliver's countrymen are warlike and vengeful and greedy. In many ways, they lack the humanity the king possesses.
Further, when Gulliver explains gunpowder to the king, that it can destroy armies, blowing up hundreds of men at once, and laying waste to cities, he offers to share the recipe, believing that a king would absolutely want such a weapon at his disposal. Indeed, most kings would. However, the king of Brobdingnag is again horrified, calling Gulliver an "impotent and groveling Insect" with "inhuman Ideas." The king has a great deal of humanity, enough to fill his whole, giant person, while Gulliver seems to have comparably little in his much smaller figure.