Fannie Mae and Freddie Mac are increasingly being challenged on their contention that they control only a relatively small share of the residential mortgage market. The latest challenge comes from a new Congressional Budget Office study that concludes Fannie Mae’s and Freddie Mac’s share of the one-to-four family mortgage market grew to 71 percent last year. CBO’s analysis, released last week, suggests the two government-sponsored enterprises don’t have to pass on to borrowers all the