From Gineipaedia, the Legend of Galactic Heroes wiki
The Humanity Congress was the chief legislative body of the United Earth Government. A representative democracy, seats in the Humanity Congress were divided unequally between Earth and her Colony Worlds, with 70% of the seats belonging to congressmen from Earth, and only 30% of the seats left for delegates from the colonies, despite the fact that the colony worlds held far greater populations. Because legislation required 70% approval in order to pass, the colony worlds we basically incapable of effecting policy in any appreciable manner.
In the later days of the United Earth Government's rule, growing discontent over unequal representation in congress as well as the continued mistreatment of the colony worlds by the Humanity Congress incited the colony worlds to rebel in the Earth–Sirius War.