In a sense, we can say that Congressional power has both expanded and contracted since the Constitution was ratified. This is true because it has expanded relative to the states while contracting relative to the presidency.
When the Constitution was ratified, the federal government as a whole was very small and unintrusive. The Congress did not make laws that touched on the daily lives of citizens all over the country. This has changed significantly in the intervening years. Today, particularly through the use of the commerce clause, the federal government (of which Congress is, of course, the legislative branch) makes laws that touch us in many ways. Antidiscrimination laws and worker safety laws are two of these. These are types of law that the Framers of the Constitution would not have imagined that Congress had the power to pass. Thus, Congressional power has expanded.
However, Congress is no longer preeminent within the federal government in the way that it was supposed to be. The Framers meant for Congress to dominate the federal government and for the president to be relatively secondary. This was the case for most of American history until the 20th century. Since then, however, presidents have taken more power to themselves. In addition, modern media have given the president much more informal power. It is now the case that presidents, and not Congress, set the agenda for the nation. This represents a contraction of Congressional power relative to the executive branch.