Look for yourself, here.
$1.2 trillion is the "remaining principal balance of the underlying mortgages". That's one third of the total balance sheet.
Acquired at the pace of $40 billion a month, it would take the Fed 2.5 years to acquire all that crappy mortgage paper, but of course the latest iteration of MBS acquisition at $40 billion a month has been going on only since last September. That means just $440 billion of the $1,200 billion has been acquired recently.
There's a whole lotta crummy paper out there, folks. And Ben Bernanke and the US Federal Reserve Bank have been buying it . . . just for you!