The biggest player in the $4 trillion mortgage-backed securities market is expected to significantly lower its profile – if not activity – in the months ahead. Fannie Mae, both the largest issuer and investor in the U.S. MBS market, was slapped this week with a 30 percent regulatory capital surcharge that observers say will force the government-sponsored enterprise to rethink the way it purchases and holds MBS. Importantly, the change is likely to impact not…