Federalist 51 which was written by James Madison concerns several important principles on which the federal government is based, separation of powers and checks and balances. Madison believes that the legislative branch is the most powerful. He states “In republican government, the legislative authority necessarily predominates.” The legislative branch is the one closest to the people in a republic and therefore would wield the most power. He probably believes this because in his own state of Virginia, the legislative branch dominated. To keep the legislative branch from becoming too powerful, he believed that “An absolute negative on the legislature appears, at first view, to be the natural defense with which the executive magistrate should be armed.” That is, the executive branch should have veto power over the legislative branch. But because the legislative branch can wield so much power, more needs to be done. The legislative branch should be further divided (into a House and Senate) and these two houses have different means of election. In the original Constitution, the House was elected directly by the people and the Senate was chosen by state legislatures.
Please note that in this essay, Madison is not talking about all three of the branches of government that we now have. Instead, he is only talking about the legislative and the executive branches. Of these two branches, he believes that the legislative will be stronger and the executive will always be weaker.
He does not really tell us why he thinks this -- he just asserts it. But the basic idea here is that it is only the legislature that has the ability to actually make laws. This makes it the stronger of the two branches.