The Founding Fathers of the United States established elasticity within the Constitution because they realized it would be dangerous not to allow for some expansion of power should the need arise. Different national leaders have used strict and loose interpretations of the Constitution in order to protect the country or to advance a certain agenda based on the mood and needs of the American public.
The Supreme Court's power has expanded as the courts have ruled in civil rights cases. The Supreme Court has ruled in cases for gay rights and whether or not corporations could give freely to political campaigns. While the courts cited precedents such as the Fourteenth Amendment and freedom of speech under the Bill of Rights, the Founding Fathers would have never imagined these issues in 1787.
Congress has passed laws concerning income taxes and protections of public lands. The legislative branch's power has grown as people now expect the government to intervene in their personal lives in order to provide social safety nets and regulations concerning conservation. This direct governmental intervention would have shocked strict constructionists as they saw the federal government as a potential threat to liberty.
The executive branch has grown as well with military interventions throughout the world authorized by the president. This is a loose interpretation of the president's role as commander-in-chief. The president also intervenes heavily in order to push his/her own political programs through Congress. The president also serves as the head of his/her national political party. The Constitution does not mention political parties but these have become an accepted part of American politics.
In many circumstances, the growth of these branches' power is just. People expect more out of the government and the growth of the branches' power reflects this. In times when a branch grows too much, such as with Johnson's use of the War Powers Act in 1964, Congress or another branch of government can regain some of its former power.