The US Fed balance sheet is composed primarily of six items; treasury holdings, MBS, liquidity swaps, revers repos, agency holdings and other loans. During the 2008 crisis the Fed had a large number of temporary programs that sometimes had large balances on them as well. Those are not shown here.
You will notice by looking at the overall Fed balance sheet during the extent of QE1 that it remained relatively unchanged. That is because the Fed had implemented liquidity swaps, loans, and reverse repos during late 2008-early 2009 (seen on the detailed balance sheet graph) and the QE1 program was put in place to effectively keep that liquidity in the marketplace as those special programs were being withdrawn. That's not the case with QE2 (and presumably with QE3).