The CEOs of Fannie Mae and Freddie Mac told lawmakers last week they have been working to manage all expenses prudently at the taxpayer-subsidized government-sponsored enterprises even as they sought to explain away reports that the two GSEs ran up a six-figure bill attending an industry convention in Chicago in October.Testifying before the House Financial Services Subcommittee on Oversight and Investigations, Fannie CEO Michael Williams and Freddie CEO Charles Haldeman noted the GSEs’ importance in the current and future mortgage markets even as they cited their efforts to reduce overall expenses through money-saving cuts and improvements in operational efficiency over the last three years.