This graphic comes from FT Alphaville, which picked it up from Credit Suisse. It shows that, as a percentage of GDP, the ECB's and BOE's balance sheets are larger than that of the U.S. The fact that the ECB's balance sheet grew more through lending than bond buying is illustrated by the second chart. If -- as many expect -- the Fed does announce another $600-$700 bln of bond (including MBS) purchases, it would boost the Fed's bottom line by about 5% of GDP, which would still put it below the ECB and BOE.
Indeed, Draghi's proposals suggest the ECB's balance sheet may also increase. The BOE is already expanding its balance sheet via gilt purchases and is expected to continue its efforts when the current program ends.