Since September 2012, the FOMC has added some $ trillion in long-term Treasuries and mortgage-backed securities to its balance sheet through this program, buying at a pace of $ billion a month in 2013. This program, known as QE3, is already twice the size of the last round of asset purchases that was initiated in November 2010, known as QE2.
It is only really banks – and in particular larger banks – that have large enough balance sheets to effectively diversify the idiosyncratic risks from SME lending, by lending to a broad enough range of firms.