GSE Portfolio Risk Reduction Act of 2011 - Amends the Housing and Community Development Act of 1992 to prohibit the Federal National Mortgage Association (Fannie Mae) and the Federal Home Loan Mortgage Corporation (Freddie Mac) (government-sponsored enterprises or GSEs) from owning mortgage assets in excess of: (1) $700 billion one year after enactment of this Act; (2) $600 billion two years after enactment of this Act; (3) $475 billion three years after enactment of this Act; (4) $350 billion four years after enactment of this Act; and (5) $250 billion five years after enactment of this Act.