The principle of "Separation of Powers" is improved with a bicameral legislature rather than a unicameral legislature because a bicameral legislature requires two bodies of government pass the laws.
For these chambers or houses, the members are elected by different methods which may vary from country to country. And by the different method of selection, the two chambers can have different number of members.
The people who established the United States were in favor of bicameral legislature as they wanted to have a management committee which is wealthier and wiser.