The Brobdingnagians are a race of huge males and females. They are literally giants, over 50 feet tall. As a result everything around them is also gigantic and Gulliver is in complete danger being that he is tiny in comparison.
This race is essentially human, going through the same feeding, breeding, and living processes as typical people. They have a peacekeeping army, engage in conflict, and understand the difference between political factions and parties. If they were normal-sized, they would not be any different than us. However, their massive size makes the mundane aspects of human physiology even more shocking as they are fully displayed at a much larger scale.
These people disgust Gulliver because he inevitably has to witness the graphic changes in their bodies, including the good, the bad, and the ugly. Yet, as people the Brobdingnagians are relatively civil, even more than us in comparison, because their laws are clear, and they seek for peace as their common goal.
The tiny Lilliputians are dramatically different in size compared to the Brobdingnagians. They are less than half a foot tall, and their world is also tiny. The system of government in the island where they inhabit is comical and ridiculous. It consists on an Emperor and the officials appointed for council, whom are elected based on ridiculous abilities.
When a great office is vacant, either by death or disgrace (which often happens,) five or six of those candidates petition the emperor to entertain his majesty and the court with a dance on the rope; and whoever jumps the highest, without falling, succeeds in the office
Lilliputians are mean and fight amongst themselves all the time. They build their own weapons and are awesome in mechanics. The politics of the Lilliputians are similar to those in England, and Swift makes sure to highlight the differences of the two main parties the way in a way that they mirror those of the Tories and the Whigs. In all the Lilliputians are tiny but their size is nothing to feel safe about. They have machinations in terms of everything, from their everyday life, to the making of war machinery, to the way that people backstab each other at court and out of court. They are little people with massive personalities.
In all, there is not just one major difference since they are two completely different races. However, for the sake of argument, we may contend that, in both races of individuals, the size of the citizens seems to contrast with their personalities. The Liliputians are small but able to cause big problems. The Brobdingnagians are large, but want to avoid those "big problems" by minimizing conflict.