All four parts of the book deal with politics and humanity. It is a social satire focused on all the horrible deeds of the English cleverly disguised as a travel journal.
Part I is the Lilliputians...the little people. They are haughty and full of themselves. They give Gulliver orders about sinking the Blefescuians (symbolic French) even though Gulliver had nothing against them. Their haughty and holier-than-though manners make it even funnier when the royal highness' quarters catch on fire and Gulliver urinates on the castle and the royal lady inside in order to save her life. That makes twice she is covered in his body fluid...the first being when he kisses her hand and the moisture from his lips coat her entire arm with slime. Hilarious! These people also do some weird things to get into office...tight rope walking and jumping and creeping. One party wears high heel shoes, the other wears low heels, and the mayor wears one of each so not to insult. Strange, but typical of politics, don't you think?
Part II is the Brobdingnagians. The giants. Here, Gulliver is a Lilliput. The King thinks of the English according to Gulliver's discussions as the most odious vermin who ever crawled on the face of the earth...filled with all the vices known to humanity--greed, pride, ambition, murder, etc. In an attempt to impress, Gulliver makes gunpowder which just blows up the place. An unflattering picture
This is a complicated question and the full answer won't fit in this space, but I will give you a short version. The entire book is a satire of many aspects of people, politics, and society. In Book 1, the Lilliputians are shown to be petty, superficial, illogical, and narrow-minded. They fight a war over a ridiculous reason. They appoint their public officials in a ludicrous way; they create titles for themselves to give themselves airs. They are small in many ways besides their stature. The Lilliputians act like they can simply pass official rulings from the government to contain the gigantic Gulliver who could easily crush every one of them if he chose. They are simply laughable in their tirades and their illusions of grandeur. Brobdingnag is in some ways, the opposite of Lilliput. Where Gulliver was 12x larger than the Lilliputians, he is that much smaller than the Brobs. The king of the Brobs looks upon Gulliver and the English the same way Gulliver looked upon the Lilliputians only to a greater degree. Gulliver's magnified view of people shows that all people are really ugly (more than in just appearances) up close; Swift's own view of people in general. Gulliver's dissertation on English government and society and the king's reaction are a put down of the English. Both places depict a dim view of humanity in general and show the ridiculousness of government.