Despite Obvious Similarities, Fannie & Freddie Befuddle Investors with Different MBS Approaches Throughout much of this year, Fannie Mae has taken heat for not growing its MBS investment portfolio as fast as rival Freddie Mac. But this week Fannie was raising eyebrows in the investment community for doing just the opposite: boosting its portfolio by more than $6 billion in April while Freddie allowed its portfolio to actually shrink by nearly $5 billion last month.