Madison answers this question most directly in Federalist No. 51, the essay where he addresses the system of checks and balances built into the Constitution.
Madison tells his readers that in "republican government, the legislative authority necessarily predominates." This is because in this kind of government "a dependence on the people is...the primary control on the government," and obviously the legislative branch is most dependent on the people. This Congress under the Constitution has the power to tax, to declare war, to borrow money, and to carry out almost all of the functions of a government. The Congress can also impeach and remove members of the other branches for a number of offenses, and the Senate has the power to give, or not give, consent to both treaties and presidential nominees.
While the Framers of the Constitution clearly wished to create a far more powerful central government than had existed under the Articles of Confederation (which only had a legislative branch)...
(The entire section contains 3 answers and 601 words.)