Since the eruption of the financial crisis, the balance sheet of the Federal Reserve System (the Fed) has increased to a factor of 5.2, from $867 billion on August 8, 2007, to $4,497 billion on December 23, 2015. This resulted from several rounds of Quantitative Easing (QE) in which the Fed bought Treasuries and other securities. Total assets have remained virtually unchanged in the past 12 months. The Fed has not reduced its holdings of Treasuries or other securities.
The US Treasury has been a beneficiary of the Fed's accumulation of securities in that the Fed earns interest on them. Each year the Fed transfers the balance of its earnings to the Treasury after deducting its own expenses. This sum has grown from $31.7 billion in 2008 to an estimated $98.7 billion in 2014.
The nearly $100 billion is free money for Congress to spend without its having to vote for a tax increase of this magnitude or, under a spending limit, enables the Treasury to borrow $100 billion less.
Some economists fear that the Fed's large balance sheet could unleash inflation when and if it sells off some of its securities into the market. Others fear that the large balance sheet may complicate efforts to stimulate the economy if it falls into recession. Who knows? I'll let the monetary economists sort that out.
What is clear is that the Treasury and Congress are happy with the now $100 billion transfers that enable less borrowing or more spending without additional taxation.